The international community sought to ease tensions after Russia and Ukraine ramped up security around the disputed Crimean peninsula.
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 Russia-backed separatists and Ukrainian forces flared up in eastern Ukraine weeks later. The conflict in the east has killed more than 9,500 people and is still raging.
Russia's FSB security service said on Wednesday it had thwarted "terrorist attacks" in Crimea this week by Ukrainian military intelligence and beaten back armed assaults, claims Kiev has fiercely denied.
Both sides have since stepped up security in the peninsula, whose annexation by Moscow in 2014 saw relations plunge to a post-Cold War nadir and served as the prelude to armed conflict between pro-Russian separatists and government troops in Ukraine's east. Poroshenko rejected the Russian claims as "fantasy" and "a provocation." Ukraine's military intelligence directorate accused Russia of boosting its forces, replenishing its munitions and building up military hardware in the war-scarred east.
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 with leaders of Ukraine, France and Germany at the sidelines of the G 20 meeting in China next month as had been planned earlier.
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 grave digger of the Minsk process." "And if he had been unaware of these decisions, it's even worse," it said.
Poroshenko, in his turn, ordered Ukrainian troops to go on combat alert not only around Crimea but also along the line of contact in eastern Ukraine, where the warring sides have continued to routinely exchange fire despite a 2015 truce. He also ordered Ukrainian diplomats to organize conversations with the U.S. and European leaders and Putin.
The recent developments have raised fears of a possible escalation of hostilities.
In Washington, U.S. State Department spokeswoman Elizabeth Trudeau voiced concern about the buildup of tensions. "Crimea is part of Ukraine, and it is recognized as such by the international community," she said, adding: "We call for the avoidance of any actions that would escalate the situation there. We believe that any actions in any way, including rhetoric, including remarks, have the ability to escalate what is already a very tense situation and a very dangerous situation."
The French foreign ministry also said Thursday that minister Jean-Marc Ayrault told Russian counterpart Sergey Lavrov that France and Germany would "continue efforts aimed at implementing the Minsk agreements in the Normandy format," which includes the four countries.
While local media and social media users have largely corroborated reports of a shootout at the Crimean border, independent accounts of the second border incident reported by the FSB were missing.
As soon as Russia forced the last Ukrainian troops based in Crimea to leave in 2014, Moscow set up fortified border crossings and sent new weapons to the peninsula – from cutting-edge fighter jets to the newest missile systems.
Despite the military buildup, hardly any disturbances let alone cross-border shootings have been reported in Crimea since the annexation.
The 2015 peace agreement for eastern Ukraine signed in Minsk has helped reduce fighting in eastern Ukraine, but peaceful settlement has floundered.
Independent Moscow-based military analyst Pavel Felgenhauer said that 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, adding: "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."
Putin menacingly referred to the Ukrainian leadership as "the people who seized power." Moscow refused to recognize Ukraine's interim authorities after Ukraine's pro-Moscow president, Viktor Yanukovych, was driven from power by massive protests, but later recognized Poroshenko.