All 31 U.S.-made M1A1 Abrams tanks promised to Kyiv by the Biden administration have arrived in Ukraine, according to the U.S. military.

Col. Martin O’Donnell, a spokesman for U.S. Army Europe and Africa, told VOA that all of the Ukrainians who trained on the tanks with U.S. forces in Germany have also returned to Ukraine, along with ammunition and spare tank parts.

“We have lived up to our end of the bargain. From this point forward, it is up to them [Ukraine] to determine when and where they will deliver this capability,” O’Donnell said.

Military officials say it could take time before the Abrams are sent to the battlefield, as Ukrainian troops make sure they have needed support elements in place and determine when and where to use the tanks for greatest effect against Russian forces.

“I think Ukraine will be deliberate in when and where they use it,” O’Donnell said. “The Abrams tank is one hell of an armored vehicle, but it’s not a silver bullet. Ultimately, it’s Ukraine’s determination to break through that matters most.”

The first of the 31 American-made Abrams tanks were delivered to Ukraine late last month, according to Ukrainian President Volodymyr Zelenskyy. The delivery came faster than initial estimates and in time for potential use in the final weeks of Kyiv’s counteroffensive against Russian forces before winter sets in.

“Abrams are already in Ukraine and are preparing to reinforce our brigades,” Zelenskyy wrote on Telegram on September 25.

The Abrams will add to other Western tanks already in Ukraine’s arsenal as it fights to reclaim Russian-held territory in Ukraine’s eastern and southern regions.

The U.S. pledge to donate Abrams tanks earlier this year came alongside a pledge from European nations to deliver German-made Leopard 2 tanks, which Berlin had been unwilling to approve without a similar commitment from the United States.

The United Kingdom was the first country to agree to send Western tanks to Ukraine, pledging its Challenger 2 tanks in January of this year, which arrived in the spring.

British Major Nick Bridges told VOA shortly after the U.K announcement that Challenger 2 tanks can “take multiple hits and stay in the fight,” even as they are considered slower than the Abrams and Leopard 2 tanks.

“The battles in the Ukraine will be slow, and what you need is a heavy tank like a Challenger [2 tank] that can take a hit, and more so than a T-72 [Russian-made tank], which will probably be destroyed after one round,” he told VOA.

Russian forces hit a Challenger 2 tank in Ukraine for what appeared to be the first time last month. Video released at the time showed a badly-damaged tank ablaze, with a Western defense source confirming to news outlets that the tank was indeed a Challenger 2 tank and that all of the crew had survived the attack.

Ukraine has asked for hundreds of Western tanks for its offensive. They have received dozens to date.

Ukraine has intensified a campaign of missile and drone strikes to hit targets deep behind Russian lines, which has placed parts of the occupied Crimean Peninsula under repeated attack.

But with winter approaching, Ukrainian forces have yet to achieve a decisive breakthrough, a concern among Kyiv’s backers that has raised questions about the future of international support.

The Abrams tanks’ arrival in Ukraine comes as the United States provided up to $200 million in additional military aid for Ukraine in a package last week. The Pentagon said the package included weapons for air defense such as the AIM-9 Sidewinder missile, artillery munitions for High Mobility Artillery Rocket Systems (HIMARS), TOW anti-tank missiles, and 155mm and 105mm rounds.

The package marked the 48th time that the U.S. has used the presidential drawdown authority to provide Ukraine’s miliary with equipment from U.S. stockpiles, and it was the first since Congress excluded new aid for Ukraine in a stopgap spending bill passed last month to prevent a government shutdown.

The U.S. has provided about $44 billion in security assistance to Ukraine since the beginning of Russia’s unprovoked invasion in February 2022.

The Pentagon still has about $5 billion of congressionally approved funding for Ukrainian military aid.

Soon after the stopgap spending bill passed, the House ousted House Speaker Kevin McCarthy from his position as speaker. The House has yet to vote for a new speaker, and new aid for Ukraine could hinge on who is selected.