WASHINGTON (AP) — The U.S. is expected to announce Wednesday that it will send about $1 billion in military aid to Ukraine, the largest single tranche of weapons and equipment since the war began, in a effort to help stall Russia’s slow but steady march to conquer the eastern Donbas region, U.S. officials said.

According to the officials, the aid is expected to include anti-ship missile launchers, howitzers and more rounds for the High Mobility Artillery Rocket Systems that U.S. forces are training Ukrainian troops on now.

The officials spoke on condition of anonymity to discuss details not yet made public.

The aid comes as U.S. Defense Secretary Lloyd Austin has convened a meeting in Brussels of more than 45 nations to discuss support for Ukraine.

At the start of the meeting, Austin warned that the West must step up weapons deliveries to Ukraine and prove its commitment to helping the country’s military fight along a 1,000-kilometer (620-mile) front line in a grinding war of attrition with Russia.

He urged the participating nations to demonstrate “our unwavering determination to get Ukraine the capabilities that it urgently needs to defend itself.” And he warned, “We can’t afford to let up and we can’t lose steam. The stakes are too high.”

Overall, since the war began in late February, the U.S. has committed about $5.6 billion in security assistance to Ukraine, including this latest package.

Officials said that about one-third of the latest $1 billion will be from presidential drawdown authority, which means the Pentagon will take weapons and equipment from it’s own stock and ship them to Ukraine.

The remaining two-thirds would be equipment and weapons purchased from industry by the U.S. and then transferred to Ukraine.

Austin’s meeting, also attended by Ukrainian Defense Minister Oleksii Reznikov, came on the opening day of a two-day gathering of NATO defense ministers at the alliance’s headquarters.

Increased arms supplies can’t come too soon for the Ukrainian forces battling to keep Russia from taking control of their country’s industrial east after more than 3½ months of war.

In his nightly address to the nation, President Volodymyr Zelensky pleaded Tuesday for more and faster deliveries of Western arms, specifically asking for anti-missile defense systems.

“Allies are committed to continue providing the military equipment that Ukraine needs to prevail, including heavy weapons and long-range systems,” said Jens Stoltenberg, NATO secretary-general.

Ukrainian Deputy Defense Minister Hanna Malyar said Tuesday that without help from the West, “we will not be able to win this war.” She said Ukraine uses 5,000 to 6,000 artillery rounds a day, while Russia uses 10 times that many.

The defense ministers also planned to discuss moves to beef up forces along NATO’s eastern flank and elsewhere, which have gathered strength since Russia invaded Ukraine.

“This will mean more presence, more capabilities and higher readiness, with more NATO forward deployed combat formations to strengthen our battlegroups in the East, more air, sea and cyber defenses, pre-positioned equipment and weapon stockpiles,” Stoltenberg said.

On a separate but related subject, he wouldn’t commit to a timeframe for Sweden and Finland joining NATO.

Turkish President Recep Tayyip Erdogan is blocking the membership bids, accusing the Nordic nations of supporting Kurdish militants deemed by Turkey to be terrorists.

“My aim is to solve this issue as soon as possible, but since we are several nations involved in this process, there is no way to tell you exactly when we will solve it,” Stoltenberg said.

Because of Turkey’s concerns, “this will take some more time than we originally expected,” he said.

Erdogan signaled Wednesday he won’t back down.

“We will most definitely not change our stance until Sweden and Finland take clear, concrete and determined steps in the fight against terrorism,” Erdogan said in an address to his ruling party’s legislators.

All 30 NATO members must agree to admit new members.

U.K. Defense Secretary Ben Wallace said at a meeting Wednesday in Oslo that a goal for the NATO summit in Madrid is ensuring “that Sweden and Finland are successfully on the next step towards accession into NATO.” But he said it’s also important to work with Turkey and mitigate its concerns.