BRUSSELS — The United States will send an additional $1 billion in military aid to Ukraine, President Biden announced Wednesday, bolstering Ukrainian forces as they are pummeled by a Russian offensive in the country’s east.

The massive arms package, which will include ammunition, artillery systems and, for the first time from the United States, mobile Harpoon anti-ship missile launchers, underscores the high stakes of Ukraine’s attempt to repel a powerful Russian assault in the Donbas region and fears that the Kremlin could still target port cities such as Odessa.

“The United States, together with our allies and partners, will not waver in our commitment to the Ukrainian people as they fight for their freedom,” Biden said in a statement after a call with President Volodymyr Zelensky. The scale of the assistance, the United States’ single largest aid package for Ukraine to date, represents an acknowledgment that the war will probably continue for many months more, generating disastrous effects for the world economy and global hunger.

The announcement came as Defense Secretary Lloyd Austin met with NATO allies and other partners for two days of talks in Brussels, focused on support for Kyiv and broader changes in transatlantic security, many of them stemming from President Vladimir Putin’s Feb. 24 invasion of Ukraine.

The surge of international support for Ukraine follows weeks of urgent appeals from Zelensky and other Ukrainian officials, who have called for sophisticated weapons, including air and missile defense systems, they say are needed to prevail against Russia’s larger, more advanced military.

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Austin, speaking at a news conference in Brussels, said the latest weapons would be “crucial” to helping Ukraine hold off Russia’s assault in Donbas. “It will make a difference,” he said. The new package brings U.S. security aid to Ukraine since the war began to $5.6 billion.

Austin suggested more aid would be coming from nations such as Germany, which he said would send long-range artillery rocket systems. Slovakia has promised helicopters and ammunition. Ukraine’s backers “will stay focused on this for as long as it takes,” he said.

Ukrainian forces, despite months of Western arms shipments, are struggling to hold off Russian forces in their Donbas offensive. The battle is now focused around the city of Severodonetsk, where civilians remain trapped beneath a chemical plant and local officials say shelling and the destruction of bridges have cut off escape routes.

Severodonetsk Mayor Alexander Stryuk said conditions for the city’s thousands of remaining civilians are growing dire as Russian forces attempt to storm Ukrainian-controlled areas from multiple directions.

British Defense Secretary Ben Wallace on Wednesday described Ukrainian troops as “exhausted,” many of them fighting for 90 days at a time, and vastly outgunned as they face off against Russia in an artillery-heavy fight. Britain, like other Western countries, is now attempting to accelerate weapons deliveries but has also stressed the need to train Ukrainian troops on new systems, which will mean a delay before they can be employed in battle.

The Donbas battle has prompted renewed speculation in Western capitals about how the war will end and what compromises, if any, Kyiv should make in any negotiated settlement. Zelensky, signaling little appetite for concession, this week vowed to retake all areas of the country controlled by Russia, including Crimea, which Moscow annexed in 2014.

French President Emmanuel Macron on Wednesday sought to telegraph strong support for Ukraine but said Kyiv must eventually negotiate. He has taken a different tack in addressing Putin’s actions than leaders like Biden, who has called the Russian leader a “killer” and a “butcher.”

“When — as I hope — Ukraine will have won, and above all when the firing has stopped, we must negotiate. The Ukrainian president and his leaders will have to negotiate with Russia,” Macron said during a visit to Romania. “And we Europeans will be at this table providing security guarantees.”

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Despite those hopes, the rift between Russia and the West has only seemed to widen as the war has ground on, amid successive waves of global sanctions and political rebukes. On Wednesday, former Russian president Dmitry Medvedev suggested in a Telegram post that Russia’s aim is the destruction of Ukraine as a nation.

“I saw a message that Ukraine, under lend-lease, wants to receive LNG [liquefied natural gas] from its overseas masters with payment for delivery in two years. Otherwise, next winter it will simply freeze,” he wrote, adding: “Just a question. Who said that in two years Ukraine will even exist on the world map?”

In response to Russia’s actions, NATO Secretary General Jens Stoltenberg said NATO members were hammering out a new assistance package and “are committed to continue providing the military equipment that Ukraine needs to prevail, including heavy weapons and long-range systems” — but did not offer specifics on plans.

Longer term, the alliance will help Ukraine’s military move from Soviet-era to NATO-standard weapons and provide the support needed to make the transition, Stoltenberg said. “This is our focus and, I think, the most urgent need in our relationship with Ukraine as we speak.”

The United States and its European allies have gradually expanded the weaponry they have provided Ukraine in recent months. But they continue to withhold certain systems, including fighter jets, and fearful that Russia might attack a NATO nation in response, they have provided only limited numbers of other arms that might be used to strike deep into Russia territory.

Ukrainian officials visiting Washington on Wednesday acknowledged that Russian forces have destroyed three of four Howitzers and some of the advanced rocket systems recently shipped by the United States. They said the West needed to accept that some equipment will be destroyed.

“Yes, we’re losing Howitzers, but again, we are at war and it’s obvious that some of this stuff will be shot down or destroyed, but unfortunately there is no other way,” said Oleksandra Ustinova, a member of the Ukrainian Parliament.

The group, which included David Arakhamia, Ukraine’s chief negotiator with Russia, said the loss of weaponry only exacerbated the need for the United States to send more equipment. “We need weapons, weapons, weapons,” he said.

It’s unclear whether the current package will satisfy Kyiv’s demands. The officials also called for the United States to provide large long-distance Gray Eagle drones.

“We keep hearing, ‘If we give you MiGs they will be shot down. If we give you drones, they will be shot down,’ ” Ustinova said. “You have to understand: We are at war. Yes, we will lose some of these.”

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Wednesday’s announcement kicked off two days of meetings in Brussels focused in large part on how the war is transforming transatlantic security. The discussions, on issues including defense spending and potential changes to NATO’s force posture, precede a June 29 summit of alliance leaders in Madrid.

“We will now take decisions on the scale and design of our posture for the longer term,” Stoltenberg said. “This will mean big increases in our presence, capabilities and readiness.”

Although Stoltenberg declined to get into specifics, he said the alliance was discussing how best to bolster its defense and deterrence, particularly on its eastern flank. This is likely to include additional forces, pre-positioning of heavy equipment and pre-assigning forces to specific countries, he said.

Overshadowing preparation for the summit is Turkey’s opposition to bids by Sweden and Finland to join the alliance. Although Stoltenberg and other leaders initially expressed confidence that NATO would move swiftly, pushback from Ankara has raised fears of a stalemate.

On Wednesday, Stoltenberg acknowledged that the opposition took him by surprise: “We didn’t have information that would be a problem,” he said. Turkey has objected to the Nordic nations’ rules for arms sales and their position on individuals Ankara says are affiliated with the Kurdistan Workers’ Party, or PKK.

The NATO leader said the alliance is working to resolve the issue and get both aspiring members to Madrid as “invitees.”

Also on Wednesday, relatives of two U.S. military veterans fighting in Ukraine said the men had gone missing, raising fears they were captured by Russian forces. The disappearances of Alexander J. Drueke, 39, and Andy Tai Huynh, 27, both of Alabama, highlight the role of foreigners who have volunteered to fight against Russia.

Drueke’s mother said he had told family members he was teaching Ukrainian forces how to use U.S.-made weapons. In phone interviews, both families shared accounts in which the two men had contacted them June 8 to say they would be unreachable during a multiday mission. Neither has been heard from since, they said.

Ryan, Lamothe and Hudson reported from Washington.