The US wants Russia "weakened" so it cannot invade again and Ukraine can win the war if it has the right equipment, Pentagon chief Lloyd Austin said Monday on returning from a trip to Kyiv.

"The first step in winning is believing that you can win. And so they believe that we can win," Austin told a group of journalists after the visit with US Secretary of State Antony Blinken.

The two, who met with Ukrainian President Volodymyr Zelensky, were the first high-profile US officials to visit Ukraine since the Russian invasion began on February 24.

"We believe that we can win, they can win if they have the right equipment, the right support," Austin said.

Austin and Blinken said US diplomats will begin a gradual return to Ukraine this week and announced $700 million (653 million euros) in additional military aid.

"We want to see Russia weakened to the degree that it can't do the kinds of things that it has done in invading Ukraine," Austin said.

Russia "has already lost a lot of military capability. And a lot of its troops quite frankly, and we want to see them not have the capability to very quickly reproduce that capability," he said.