Lithuania on Monday announced a new rapid reaction force designed to meet what it termed new unconventional security threats highlighted by the Ukraine crisis.

The Baltic nation's top general said 2,500 troops will be on high alert as of November to counteract so-called "hybrid warfare" involving unconventional attacks by unmarked combatants, like those in eastern Ukraine.

The move comes after NATO last month approved a rapid-response force in the wake of Russia deploying new combat tactics in Ukraine amid its March takeover of Crimea.

"We must immediately increase our readiness for unplanned military actions during peacetime," Major General Jonas Vytautas Zukas told reporters.

New threats include "manipulating national minorities, provocations, attacks by non-state armed groups, illegal border crossing, breach of military transit procedures," Zukas said.

The force, which accounts for about a third of Lithuania's 7,000-strong military personnel, will take from two to 24 hours to be fully mobilised.

The defence ministry on Monday also tabled legislation that would allow the president to authorise the use of military force in a defined territory without first declaring martial law.

So-called "little green men" -- armed soldiers without any identifying insignia -- played a key role in seizing Ukrainian military bases in Crimea after a hastily organised referendum led to its annexation by Russia in March.

Like Ukraine, Lithuania was part of the Soviet Union until 1990, but unlike it, joined the European Union and NATO in 2004 and gained protection under the alliance's Article Five collective defence guarantee.

Lithuania, along with Baltic partners Latvia and Estonia, has had rocky ties with Moscow since independence. Tensions have spiralled over Russia's role in the Ukraine crisis.