The Ukrainian government says troops have repelled an overnight raid on a base at Artemivsk, between Donetsk and Slaviansk, in eastern Ukraine on Thursday.
A soldier was wounded in the attack by about 70 people who Interior Minister Arsen Avakov said on social networking site Facebook were led by Russian soldiers.
Mr Avakov said pro-Russian protesters had left the town hall in Mariupol, an industrial city on the Black Sea coast, and the mayor was back in his office - meeting agreements made with Russia at a Geneva meeting a week ago, Reuters reports.
Moscow denies having any troops in Ukraine.
Meanwhile, the first contingent of American troops has landed in Poland for military exercises amid tensions with Russia over Ukraine. An initial 150 soldiers are to be followed by a further 450 within days.
US President Barack Obama has warned Russia it faces new sanctions if it refuses to implement the agreement to reduce tensions in eastern Ukraine.
The troops are expected to be carrying out military exercises in Poland as well as in Lithuania, Latvia and Estonia for the coming months, the BBC reports