Russian President Vladimir Putin on Thursday called for security to be ramped up on the annexed Crimea peninsula after accusing Ukraine of attempting armed incursions into the disputed region.

Russian President Vladimir Putin on Thursday called for security to be ramped up on the annexed Crimea peninsula after accusing Ukraine of attempting armed incursions into the disputed region.

Russia's security agency on Wednesday said it had thwarted "terrorist attacks" in Crimea by Ukrainian military intelligence and beaten back armed assaults, but Kiev furiously denied the incidents.

Putin held a meeting with his security chiefs to discuss "additional measures for assuring security for citizens and essential infrastructure in Crimea," the Kremlin said in a statement.

"Scenarios were carefully considered for anti-terrorist security measures at the land border, in the waters and in the airspace of Crimea," it said.

The FSB security service said Wednesday that one of its officers was killed in armed clashes while arresting "terrorists" on the night of August 6-7, while a Russian soldier died in a firefight with "sabotage-terrorist" groups sent by the Ukrainian defense ministry on August 8.

An irate Putin then accused the authorities in Kiev of "practicing terror" and warned the deaths of the two Russian officers would have consequences.

"We obviously will not let such things slide by," Putin said. "This is a very dangerous game."

Ukrainian President Petro Poroshenko hit back that Moscow's claims were "senseless and cynical".

"These fantasies are only another pretext for the next military threats toward Ukraine," Interfax-Ukraine news agency reported him as saying.

Moscow and Kiev have been locked in a bitter feud since the Kremlin annexed  Crimea following a national referendum after Russian-backed president Viktor Yanukovych was ousted.

The crisis has sent ties between Moscow and the West to their lowest point since the Cold War.