Ukrainian President Petro Poroshenko says he has instructed all military units near Russian-occupied Crimea and in the easterly Donbas region to be at the highest level of combat readiness, following Russian allegations of a Ukrainian incursion into Crimea.

Also on August 11, the spokesman for Ukraine's General Staff told Reuters that Ukraine had been holding scheduled military exercises in southern Ukraine since August 10.

The statements come as Kyiv says Russia has amassed more troops in recent days equipped with more modern equipment on Ukraine's border with Russian-annexed Crimea

Oleh Slobodyan, the spokesman for the Ukrainian border guards, said on August 11 that "we can unequivocally say that Russian troops who were there since March are now being replaced with others."

"These troops are coming with more modern equipment and there are air assault units. In recent days, we see a strengthening of the units that are at the border. Their number increased," Slobodyan said.

Russian President Vladimir Putin accused Kyiv on August 10 of trying to destabilize Russian-annexed Crimea through saboteurs. Kyiv has rejected the charges as "senseless and cynical".

Based on reporting by Reuters and AP