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

Jens Stoltenberg said that while NATO is not back on Cold War terms with its former arch-enemy, recent Russian behaviour 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. NATO pilots have conducted over 100 intercepts of Russian aircraft this year, about three times more than in 2013, the alliance's military officials said Wednesday.

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

"We need to keep our forces ready, therefore we are investing in high readiness, new capabilities," Stoltenberg told a press conference. "We are ... increasing air policing as an answer to the increased air activities we are seeing from Russia."

Ian Litschko, a Russia analyst at the NATO Council of Canada, said the uptick in Russian flights appears to be a "show of force" by President Vladimir Putin and his generals to demonstrate to NATO "that we're still here, we're still relevant."

German Chancellor Angela Merkel told reporters in Berlin Thursday it was clear the Russians were closely watching European airspace, but that she had no concerns so far.

"Over the past few months, I have been seeing more intense activity when it comes to Russian armed forces. But at this point, I am not worried that there is major violation of our airspace," Merkel said.

In Athens, Stoltenberg 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.

Associated Press writers John-Thor Dahlburg and Raf Casert in Brussels and Frank Jordans in Berlin.