Russian army in Ukraine disturbs Bishop

Foreign Minister Julie Bishop says she's gravely worried about reports of a Russian military presence in Ukraine.

Her comments came amid reports of masked troops in Crimea, including at its international airport, and after the Australian government increased its travel warning for the strife-torn nation, with tourists urged to exercise extreme caution.

"We are deeply concerned about the instability in Ukraine," Ms Bishop told AAP.

"We are disturbed by reports overnight that there is activity in the Russian military on the borders.

"So we call for calm. We call for a political solution and we hope that any Australians seeking to travel reconsider the necessity of doing so."