Top US general says indicators of Russian military moves are worrying

The United States is tracking enough indicators and warnings surrounding Russian military activity near Ukraine to trigger "a lot of concern" and Russian rhetoric appears increasingly strident, the top U.S. military officer said on Thursday. 

Army General Mark Milley, chairman of the Joint Chiefs of Staff, declined to speculate about the kinds of options the United States might consider in the event of a Russian invasion. 

But Milley stressed the importance of Ukraine's sovereignty to Washington and to the NATO alliance. 

"There's significant national security interests of the United States and of NATO member states at stake here if there was an overt act of aggressive action militarily by the Russians into a nation state that has been independent since 1991," Milley said during a flight from Seoul to Washington.