Russia has responded to the killing of five pro-Russian fighters in Ukraine on Thursday by mounting military drills close to their joint border.

The Russian military held the drills in the Krasnodar region close to Ukraine, setting up a competition for jet fighter pilots.

Russian President Putin said it would be a “very serious crime” if Ukraine was using its army on its own people and threatened “consequences.”

Ukraine’s Interior Ministry said pro-Russian militants had been killed by Ukrainian forces who were demolishing illegal checkpoints near the eastern city of Slovyansk.

Flexing their military muscles in return, US troops flew into Latvia’s capital Riga on Thursday for a military drill, adding to US forces which had arrived in Poland the previous day.

The United States has said it won’t intervene militarily in Ukraine but wants to reassure it’s NATO allies. Estonia, Latvia and Lithuania all have Russian speaking minorities and are increasingly concerned after Russia annexed Crimea from Ukraine.