Moscow has played up footage showing German Leopard tanks and US Bradley fighting vehicles it claims were captured in the assaults.

"I think the Russians have shown us that same five vehicles about 1000 times from 10 different angles," Austin said after a meeting of Ukraine and its Western backers in Brussels.

"But quite frankly, the Ukrainians still have a lot of combat capability, combat power."

Austin said that "this is a war, so we know that there will be battle damage on both sides" and more important was Kyiv's ability to repair damaged equipment.

"This will continue to be a tough fight as we anticipate it, and I believe that the element that does the best in terms of sustainment will probably have the advantage at the end of the day," he said.

"Our focus is on making sure that we continue to push forward what Ukraine needs in order to be successful."

Top US General Mark Milley said it was still far too early "to put any estimates" on how long the Ukrainian counter-offensive could last.

"This is a very difficult fight, it is a very violent fight and it will likely take a considerable amount of time and at high cost," the chairman of the Joint Chiefs of Staff said.

Ukraine's Western supporters -- spearheaded by the United States -- met Thursday at NATO headquarters in Brussels to pledge fresh weapon deliveries for Kyiv.

Ukrainian Defence Minister Oleksiy Reznikov said there was "very important news" on the progress on the coalition of countries willing to offer training for Kyiv's pilots on F-16 fighter jets.

Dutch minister Kajsa Ollongren said the Netherlands, which has taken the lead on the push, hopes to start the training by this summer.

She said there was then the intention to eventually create a training centre in an eastern European country neighbouring Ukraine.