NATO said Thursday it was watching "with concern" growing tensions between Ukraine and Russia over Crimea, and urged Moscow to defuse the situation.

Ukraine earlier put its forces on high alert after Russia accused it of "terrorist attacks" into Crimea, which Moscow annexed in 2014.

A NATO official said the US-led military alliance was "monitoring closely and with concern the heightened tensions between Russia and Ukraine."

The official, who asked not be named, told AFP "Russia's recent military activity in Crimea is not helpful for easing tensions.

"We call on Russia to work for calm and de-escalation."

The UN Security Council was to discuss the situation later Thursday at Ukraine's request, after pro-Western President Petro Poroshenko met his top military and put them on a heightened state of alert along the frontier with Crimea.

Russian President Vladimir Putin meanwhile held a meeting with his security chiefs.

Ukraine and Russia have been locked in a bitter dispute since the ouster of pro-Moscow president Viktor Yanukovych in early 2014, with peace talks getting nowhere despite several ceasefires.

Fighting between Ukraine government troops and pro-Moscow rebels in the east of the country had been relatively low level in recent months in a conflict that has left more than 9,500 dead.

The NATO official said the alliance was also "deeply concerned" by a recent upsurge in fighting.

NATO leaders meeting in Warsaw last month reiterated their support for Ukraine's territorial integrity and insisted again they would never recognise Russia's annexation of Crimea.