Ukrainian President Petro Poroshenko put his troops on the border with Crimea and in the country’s eastern regions on “high alert” after warning that Russia is seeking to reignite conflict in the disputed territories.

The command came a day after Russian President Vladimir Putin said Ukrainian agents had engaged in “terror” on the Black Sea peninsula, which he seized in 2014. Putin vowed to respond with “very serious” measures, touching off the worst diplomatic standoff between the two countries since a truce signed last year in Belarus eased heavy fighting in Ukraine’s conflict with pro-Russian separatists.

“There may be escalation in eastern Ukraine and that is very dangerous,” Alexei Makarkin, deputy head of the Moscow-based Center for Political Technologies, said by phone on Thursday. “The events are developing according to a pretty negative scenario. Neither side has any trust in the other.”

The confrontation coincided with a surge in violence in Ukraine’s eastern Donbas territories and torpedoed plans to revive four-way peace talks at the September G-20 meeting in China, with Putin reversing earlier support and calling the negotiations “pointless.”

The timing of the flareup perplexed military and political analysts, as it followed efforts by Putin to break Russia’s international isolation, including a push to repair ties with Turkish President Recep Tayyip Erdogan that frayed after Turkey shot down a Russian fighter on the Syrian border last year.

The yield on Ukraine’s dollar-denominated note maturing in 2019 jumped 40 basis points from a record low to 7.87 percent at 3:06 p.m. in Kiev.

Putin discussed bolstering defenses on the peninsula with his Security Council, the Kremlin press service said on Thursday. A day earlier, he said the FSB, the Federal Security Service that’s the main successor of the Soviet-era KGB, reported Ukrainian intelligence officers killed two Russian servicemen during covert operations in Crimea. He added that he “certainly won’t let such things pass by.”

Poroshenko dismissed Putin’s accusations as “fiction” that could be an “excuse for further military threats” by Russia. He ordered the armed forces, national guard and border troops to go on high alert on Thursday and urged police to step up security to prevent potential terrorist attacks, according to a statement on his website.

The country of more than million people has accused its former fellow Soviet republic of funneling cash, weapons and fighters to the separatists who have seized control of much of its easternmost regions in a conflict that the United Nations estimates has killed at least 9,500 people.

Ukraine’s United Nations mission called for “urgent” Security Council consultations over Russia’s comments. NATO said it was closely monitoring the heightened tensions between Russia and Ukraine, and both it and the U.S. said they had seen no evidence corroborating Russia’s allegations.

“Russia has not provided any tangible evidence for its accusations against Ukraine,” the North Atlantic Treaty Organization said in an emailed statement in Brussels. “We are also deeply concerned by the recent upsurge in violence in eastern Ukraine, and the increase in cease-fire violations along the line of contact, primarily by the Russian-backed militants.”

Poroshenko ordered the Foreign Ministry to organize phone calls with Putin and U.S. Vice President Joe Biden. He also sought to speak with other leaders, including German Chancellor Angela Merkel, whose Foreign Ministry called the events in Crimea “worrying.”

The Russian leader has exhibited a tendency to use instability in the region as leverage in negotiations. He also has a track record of launching military operations while the world’s attention is on the Olympic Games. The annexation of Crimea, for example, came just after Russia hosted them in Sochi, and Russia sent troops into Georgia during the Beijing Olympics in 2008. This year’s Games have been marked by a partial ban on Russian athletes by doping authorities, which Putin said was an unfair decision made by “short-sighted politicians.”

“It is always difficult to read Putin’s military game plan on the ground,” Tim Ash, a strategist at Nomura International, said in a note. “This could be much of the same as we have seen over the past year, of small re-escalations just to impress on the West, and policy makers in Kyiv, that the issue over Crimea/Donbas and Russo-Ukraine relations still is unresolved, and Moscow remains a key player.”

Western countries have refused to recognize Russia’s takeover of Crimea and have imposed sanctions that have helped force the world’s biggest energy-exporting economy into recession. Putin’s comment on the futility of further talks tied to the cease-fire signed in Minsk may suggest he’s threatening to reignite the conflict.

“The Minsk process is practically dead,” said Evgeny Minchenko, head of Minchenko Consulting in Moscow. “The funeral of the Minsk process is coming faster now. That’s for sure.”

Other motivations could include bolstering domestic support before Russia’s Sept. 18 parliamentary elections, providing a distraction before Ukraine’s 25th anniversary of independence from the Soviet Union on Aug. 24, or maneuvering before the U.S. presidential elections in November, Ash said.

Washington’s ambassador to Ukraine, Geoffrey Pyatt, said on Twitter that Russia has a record of levying false accusations at Ukraine to deflect from its own “illegal” actions, and that the U.S. would maintain its Crimea-related sanctions against Russia until it returns the peninsula to Kiev’s control.

One soldier was killed and four were wounded over the past 24 hours, military spokesman Oleksandr Motuzyanyk said on Thursday. He said Russia was engaging in “yet another step in the hybrid war against Ukraine.”