Washington - The top U.S. military officer is warning that the war in Ukraine will likely last years, raising concerns that the world 'is becoming more unstable and the potential for significant international conflict between great powers is increasing, not decreasing.'

During testimony in front of the House Armed Services Committee on Tuesday, Chairman of the Joint Chiefs of Staff General Mark Milley called Russia's invasion of Ukraine 'the greatest threat to peace and security of Europe, and perhaps the world, in (his) 42 years of service in uniform.'

'The United States is at a very critical and historic geostrategic inflection point. We need to pursue a clear-eyed strategy, maintaining the peace with the unambiguous capability of strength relative to China or Russia,' he said, referring to the top two potential military threats to the U.S.

Asked by lawmakers what could have stopped Russian President Vladimir Putin from attacking Ukraine, Milley said the only defense possible may have been to put U.S. forces inside the country, which he would not advise because it would have risked an armed conflict with Russia.

'Candidly, short of the commitment of U.S. military forces into Ukraine proper, I'm not sure he (Putin) was deterrable. This has been a long-term objective of his that goes back years,' Milley said.

Defense Secretary Lloyd Austin added that Russian demands, which include a ban on Ukraine entering NATO and a limit to NATO's deployments of troops and weapons on its eastern flank, were unacceptable.

Secretary of Defense Lloyd Austin speaks during a House Armed Services Committee hearing on the fiscal year 2023 defense budget, in Washington, April 5, 2022.

He and Milley confirmed that while U.S. forces were not training Ukrainian forces inside Ukraine or in neighboring Poland, they were training them outside of Ukraine, including inside the United States.

The U.S. continues to talk to Ukraine on a 'daily basis,' and Ukrainians have used U.S. military aid, including Stinger and Javelin missiles, effectively against Russian troops and weapons, Austin told lawmakers.

'We look for things that also can provide them an advantage in this fight, and you've seen us begin to deploy some of those things,' he said.

In addition to training Ukrainians, the U.S. is looking at ways to provide additional military training to non-NATO allies such as Georgia and Finland, Austin said.

Milley told lawmakers that NATO countries on the alliance's eastern flank, such as Poland and Romania, are 'very willing' to establish permanent U.S. bases.

'They'll build them. They'll pay for them,' Milley said, adding that U.S. forces could 'cycle through on a rotational basis' to 'get the effect of a permanent presence of forces' without asking U.S. troops to commit to two- or three-year deployments.