Defence ministers for the three Baltic states said Thursday they are mulling a joint air defence system in response to security concerns over Russia's activity in the region.

The ministers of NATO-members Estonia, Latvia and Lithuania met in the northern Lithuanian city of Panevezys along with their Ukrainian counterpart, at a time when their relations with Russia are at a post-Soviet low over the Ukraine crisis.

"We plan to analyse the possibility of developing a medium-range air defence system to strengthen our defence capabilities," Lithuanian Defence Minister Juozas Olekas told reporters.

"External threats lead us to cooperate more," he said, referring to neighbour and Soviet-era master Russia, which has spooked the region with its intervention in Ukraine.

NATO has been guarding the skies over the three small Baltic states since 2004, when they joined the defence alliance but lacked the air power to monitor their own airspace.

Under Moscow's thumb until 1991, the Baltic trio has been on high alert since Russia annexed the Crimean peninsula from Ukraine last year.

Pro-Russian separatists have since been battling Ukrainian government forces for over a year in a conflict that has claimed nearly 6,300 lives and left well over a million people homeless.

"The situation remains complicated and absolutely unpredictable," Ukrainian Defence Minister Stepan Poltorak told reporters.

He added that since the inking of a fragile February ceasefire agreement 100 of their soldiers have died and 500 have been wounded.

"We consider there is a high risk of further escalation of the situation."

The Baltics said earlier this month that they will formally ask the NATO defence alliance to deploy several thousand permanent troops in their region as a deterrent to Russia.