The Latest on the Crimean border dispute between Ukraine and Russia (all times local):

5:10 p.m.

A NATO official says the U.S.-led alliance is deeply concerned by rising tensions between Russia and Ukraine and is monitoring the situation closely.

The official, who wasn't authorized to make statements on the record and spoke on condition of anonymity, called on Moscow to "work for calm and de-escalation." The official said Russia had provided no proof of its accusation that Ukraine sent saboteurs to carry out attacks in Crimea.

The NATO official said Thursday that "Russia's recent military activity in Crimea is not helpful for easing tensions."

The official urged all parties "to return to the negotiating table and work toward a peaceful settlement of the conflict in Ukraine by diplomatic and peaceful means.

--By John-Thor Dahlburg in Brussels.

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2:50 p.m.

Ukraine's president has ordered the army to be on combat alert on the country's de-facto border with Crimea and on the front line in eastern Ukraine following Moscow's accusations that Ukraine sent in "saboteurs" to carry out attacks in Crimea.

Russia annexed the Black Sea peninsula from Ukraine in March 2014 following a hastily called referendum. The move sparked Russia-backed separatists to begin fighting government forces in eastern Ukraine, where deadly battles are still ongoing.

The Russian intelligence agency FSB on Wednesday said one of its officers and an army soldier were killed over the weekend in two separate incidents while fending off what Moscow described as a series of attacks by Ukrainian "saboteurs."

Ukraine rejected the claims as "fantasy" and "a provocation."

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11:10 a.m.

Russian President Vladimir Putin has held a meeting of the country's top brass to discuss boosting security in Crimea following the reports of foiled terrorist attacks.

The Russian intelligence said Wednesday that two people were killed while fending off what it described as a series of attempted attacks in Crimea by Ukrainian "saboteurs." Ukraine rejected the claims as "fantasy" and "a provocation."

Russia annexed the Black Sea peninsula in March 2014 following a hastily called referendum, an event that sparked Russia-backed separatists to begin fighting in eastern Ukraine, where deadly fighting is still ongoing.

The Kremlin on Thursday said Putin chaired a Security Council session to discuss "additional measures" to ensure security at Crimea's de-facto border, the territorial waters around it and the Crimean airspace.