Ukraine has repeatedly pushed for modern Western tanks to help battle Russia's invasion, and Germany has reportedly linked its decision on whether to do so to Washington's willingness to provide Abrams.

"I just don't think we're there yet," US Undersecretary of Defense for Policy Colin Kahl told journalists when asked about providing Abrams to Ukraine, though he did not completely close the door on a shift in the future.

"The Abrams tank is a very complicated piece of equipment. It's expensive, it's hard to train on, it has a jet engine -- I think it's about three gallons to the mile with jet fuel. It is not the easiest system to maintain," Kahl said.

He also noted that US defense chief Lloyd Austin has been keen to avoid providing equipment to Ukraine that "they can't repair, they can't sustain and that they over the long term can't afford because it's not helpful."

Both German and US media reported that Berlin would not agree to provide Leopard tanks to Ukraine unless Washington made a similar decision, but Kyiv has received pledges of a variety of other armored vehicles.

Britain promised last weekend that it would provide Challenger 2 tanks to Ukraine, becoming the first country to announce the provision of Western heavy tanks to Kyiv.

Germany and the United States said last week that they would respectively provide Bradley and Marder armored vehicles, and France has also promised to provide AMX-10 RC light tanks.

Asked about the thinking behind the rise in the number of armored vehicles being promised to Kyiv, Kahl said they are needed to help Ukrainian forces defeat increasingly heavy Russian defenses.

Moscow's troops are "digging in, they're digging trenches, they're putting these Dragon's Teeth, laying mines. They're really trying to fortify that FLOT, that forward line of troops," he said.

The aim is to "enable them to change that dynamic with these kind of static defenses by being able to fire and maneuver through the use of more mechanized forces," Kahl said.