Baltic countries and Poland fear that the size of the NATO force is too minuscule to efficiently deter Russian aggression in the region.

When asked about the likelihood of Russian aggression in the Baltics, Lithuania's Defence Minister Juozas Olekas told Reuters: "We cannot exclude it ... They might exercise on the borders and then switch to invasion in hours."

Olekas may raise the matter with NATO colleagues at the meeting of ministers on Tuesday and Wednesday in Brussels.

"We need to stop possible air aggression," said Olekas. "We are discussing creating a regional medium-range air defense system together with the Latvians, the Estonians and the Poles."

Lithuania, Latvia and Estonia have kept their armies on alert, making it easier for mobilisation in the situation of a possible conflict with Moscow.

Russia's annexation of Crimea in 2014 is still fresh in the memories of the former Soviet-bloc states, who are seeking military help from their Western allies to build an air defence system against aircraft and missiles.

Analysts say this move could further escalate tensions as the move will surely be condemned by Moscow as yet more evidence of a NATO strategy threatening its borders.

NATO defence ministers are set to agree this week on a new multinational force. The United States, Germany and Britain are set to lead battalions of about 1,000 troops each. Canada may lead a fourth.