NATO's new chief says the alliance is increasing its readiness and air policing following a spike in Russian air force activity in Eastern Europe.

Jens Stoltenberg says while NATO is not back on Cold War terms with its former arch-enemy, recent Russian behavior has severely undermined the trust built up over decades.

Tensions have been running high between NATO and Russia since Russia annexed the Crimean Peninsula from Ukraine in March.

Speaking Thursday during a visit to Athens, Stoltenberg said the trans-Atlantic military alliance "remains vigilant" in view of this year's tripling of Russian military flights along NATO's eastern borders.

He also urged Russia to remove its forces from Ukraine and warned against plans by pro-Russian separatists to hold local elections in eastern Ukraine. Russia denies having forces in Ukraine.