MOSCOW — Ukraine put its troops on combat alert Thursday along the country's de-facto borders with Crimea and separatist rebels in the east amid an escalating war of words with Russia over Crimea.

Ukrainian President Petro Poroshenko issued the order after Moscow accused his country of sending several groups of “saboteurs” to carry out attacks in Crimea and said that two Russians died while fending off their incursions. Ukraine has denied the claim.

Russia annexed Crimea from Ukraine in March 2014 following a hastily called referendum, and a conflict between Russian-backed separatists and Ukrainian forces flared up in eastern Ukraine weeks later.

Russia's domestic security agency, the FSB said in Wednesday's statement that one of its officers was killed in a gunbattle with “saboteurs” from Ukraine over the weekend. The agency also said that two more groups tried to force their way into Crimea early Monday, supported by Ukrainian artillery and armor. One Russian army soldier died in that clash, the FSB said.

Russian media reports say at least five members of a sabotage group were captured.

One of the captured suspects, identified as Yevgeny Panov, told FSB investigators in remarks released by Russia's REN TV television that he was part of a group preparing to conduct acts of sabotage at a ferry crossing, an oil depot and a chemical factory and to blow up Russian military equipment.

He said the group was directed by Ukrainian military intelligence and included some of its officers.

Poroshenko rejected the Russian claims as “fantasy” and “a provocation.”

Russian President Vladimir Putin denounced what he described as Ukraine's “stupid and criminal” action and called a session of his Security Council on Thursday to discuss boosting security in Crimea.

He also said Wednesday that it makes no sense to discuss the implementation of the Minsk peace deal for eastern Ukraine at the sidelines of the Group of 20 meeting in China next month.

The Russian Foreign Ministry followed up Thursday by warning that if Poroshenko had been involved in “criminal decisions to stage armed provocations” in Crimea, “he could claim the role of the gravedigger of the Minsk process.”

“And if he had been unaware of these decisions, it's even worse,” it said.

Poroshenko ordered Ukrainian troops to go on combat alert not only on the de-facto border with Crimea but also along the line of contact in eastern Ukraine, where the warring sides have routinely exchanged fire despite a 2015 truce.