WASHINGTON (Reuters) -U.S. Defense Secretary Lloyd Austin defended America's response to the war in Ukraine on Tuesday during a heated exchange with a Republican lawmaker who accused the Pentagon of over-estimating Russia's military capability.

"Has it occurred to you that Russia has not overrun Ukraine because of what we've done? And our allies have done? Have you ever even thought about that?," Austin asked rhetorically to Rep. Matt Gaetz during testimony to the House Armed Services Committee. Austin and other U.S. officials say U.S. support to Ukrainian forces, along with Ukraine's strong will to fight, has thwarted Russia's plans for a swift victory in a campaign now in its sixth week.

General Mark Milley, the chairman of the Joint Chiefs of Staff, said that the conflict in Ukraine could last for years.

"I do think this is a very protracted conflict and I think it is at least measured in years," Milley told lawmakers during the same hearing.

He added that "NATO, the United States, Ukraine and all of the allies and partners that are supporting Ukraine are going to be involved in this for quite some time."

Russian-backed separatists have already been fighting in eastern Ukraine since 2014, a conflict which has killed about 15,000 people.