"Putin thought that he could very rapidly take over the country of Ukraine, very rapidly capture this capital city. He was wrong," Austin told a hearing of the Senate Armed Services Committee in Congress.

"I think Putin has given up on his efforts to capture the capital city and is now focused on the south and east of the country," said Austin.

But the path of the overall war, six weeks after Russia invaded Ukraine, remains uncertain, the chairman of the Joint Chiefs of Staff, General Mark Milley, told the same hearing.

For Ukraine to "win" the fight, it needs to remain a free and independent nation, with its recognized territory intact, he said.

"That's going to be very difficult. That's going to be a long slog," Milley said.

"The first part of it has probably been successfully waged," he said of the war that began on February 24.

"But there is a significant battle yet ahead down in the southeast, down around the Donbas region where the Russians intend to mass forces and continue their assault," he said.

"So I think it's an open question right now, how this ends."

Austin told the panel of lawmakers that the United States is providing intelligence to Ukraine's military to support its fight in the Donbas, where Moscow-backed secessionists have been fighting government forces since 2014 and now have the direct support of Russian troops.

But Milley said the fight in that area will be difficult, and that to try to push the Russians out, Ukraine will likely need more arms support, like tanks.

"The fight down in the southeast -- the terrain is different than it is in the north," Milley explained.

"It is much more open and lends itself to armor, mechanized offensive operations, on both sides. And so those are the systems that they're looking for," he said.

"They are asking for and they could probably use additional armor and artillery," he said.