By NATALIYA VASILYEVA

MOSCOW — Ukraine’s president put his army on combat alert Thursday along the country’s de-facto borders with Crimea and separatist rebels in the east as a war of words between Russia and Ukraine threatened to heat up the largely frozen conflict over the Black Sea peninsula.

Ukrainian President Petro Poroshenko issued the order after Moscow accused his country of sending in “saboteurs” to carry out attacks in Crimea.

Russia annexed Crimea from Ukraine in March 2014 following a hastily called referendum, a move that sparked fighting between Russia-backed separatists and government forces in eastern Ukraine. The conflict in the east has killed over 9,500 people and is still ongoing.

The Russian intelligence service FSB issued a statement Wednesday, saying one of its officers was killed over the weekend a few kilometers from the de-facto border between Crimea and Ukraine after a gun battle with a group of “saboteurs” from Ukraine.

The FSB said the intruders carried an arsenal of bombs, ammunition and mines. They also reported another alleged incident in which 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.

Ukraine rejected the claims as “fantasy” and “a provocation.”

Russian President Vladimir Putin upped the ante Thursday morning when he directly accused the Ukrainian government of plotting the attacks and called a meeting of the country’s top brass to discuss boosting security in Crimea following reports of the foiled attacks.

Within hours, Poroshenko ordered the Ukrainian army to go on combat alert not only on the de-factor border with Crimea but also along the line of contact in eastern Ukraine, where both sides were supposed to have withdrawn heavy weaponry but have been sporadically using them, according to international monitors.

The reports by Russian intelligence about foiled attacks in Crimea suggest the Kremlin is looking for a pretext to up the ante in its largely dormant confrontation with Ukraine. While local media and social media users have largely corroborated reports of a shootout at the Crimean border, there seemed to be no independent accounts of the second incident reported by the FSB, in which the Ukrainian army allegedly used artillery to cover saboteurs who tried to enter Crimea.

“A pretense of an anti-terrorism operation staged by Russia is more plausible than an actual Ukrainian attack on Crimea,” prominent Russian journalist Oleg Kashin wrote Thursday on Slon.ru. “Russia deliberately pushes for an escalation and ignores the opportunities it has to preserve the status quo.”

Peace talks in the Belarusian capital of Minsk in 2015 helped ease the fighting in eastern Ukraine but did not solve the crisis. While the separatists never allowed Ukrainian forces to regain control of the border with Russia as agreed, Ukraine has also not adopted legislation to provide broad autonomy to these territories. However, neither side has recently raised any alarm about the Minsk peace accords not being implemented, showing that both Russia and Ukraine find the current stalemate acceptable.

Military analyst Pavel Felgenhauer says if the Kremlin wanted to make gains or solidify its positions in Ukraine, now would be a good moment because the world’s attention is elsewhere.

“The temptation is high to try and use this occasion to solve the Ukrainian problem once and for all,” he said. “While America is right now not very operational because it’s in a midst of a divisive election campaign, Europe is also divided — on Brexit, on refugees, on sanctions against Russia.”

As soon as Russia forced the last Ukrainian troops based in Crimea to leave in 2014, Moscow began setting up fortified border crossings and sending weapons to the peninsula — from cutting-edge fighter jets to the newest missile defense systems.

Up until now, the de-facto Crimean border and personnel bore no resemblance to the trigger-happy fighters on both sides of the front line in eastern Ukraine. Hardly any disturbances or let alone cross-border shootings have been reported in Crimea since the annexation.

In an odd detail, Putin on Wednesday referred to Ukrainian authorities as “the people who seized power.” In 2014, Moscow rejected recognizing the Ukrainian opposition’s interim government, which came in after Ukraine’s pro-Moscow president, Viktor Yanukovych, fled to Russia, saying instead that was a coup. But after Poroshenko was elected in May 2014, Putin began to call the Ukrainian government “partners.”

The change of language could be a signal for a Russian change of plan regarding Ukraine.

“The main issue is what is going to happen to the Minsk talks — whether Russia will stop them or start asking for more concessions,” respected daily Vedomosti said in an editorial Thursday. “In his rhetoric, Putin has returned to 2014, when he did not consider the Ukrainian government legitimate.”