BRUSSELS, Belgium -- U.S. Defence Secretary Chuck Hagel urged Russia on Thursday not to take any action on Ukraine that might boost tensions or be subject to misinterpretation.

On Wednesday the Russia military announced large-scale military exercises near Ukraine's borders. On Thursday, unidentified armed men occupied a government building in Crimea.

"These are times for cool, wise leadership on Russia's side and everyone's side," Hagel told reporters in Brussels after a NATO defence ministers meeting. He said there are concerns that Russia could act in a way that would lead to miscalculation during a "delicate time" in Ukraine.

"Until we know more details, what really happened, who's in charge, the focus should be on let's keep the tensions down, let's see no provocative actions by anyone, any military," Hagel said.

The defence secretary said the United States strongly supports Ukraine's territorial integrity and is closely monitoring the Russian manoeuvrs. He said the Obama administration expects all nations to respect Ukraine's sovereignty and to avoid provocations.

Hagel said his staff was trying to set up a phone call for him with his counterpart in Moscow, Russian Defence Minister Sergei Shoigu, in the next day or two, and that U.S. Secretary of State John Kerry also would be speaking to Russian Foreign Minister Sergei Lavrov.

"We have other connecting points, and we're talking with the Russians through other government-to- government channels," Hagel added.

He and other NATO ministers met earlier Thursday with Ukrainian officials. NATO Secretary-General Anders Fogh Rasmussen said Russian authorities had informed the U.S. alliance of the military exercises, and "made it clear" they have nothing to do with Ukraine, whose pro-Moscow president has fled to Russia and been replaced by a leadership friendlier to the West.

"Having said that, obviously it doesn't make things easier that there is a coincidence between the timing of this exercise and the ongoing events in Ukraine," Rasmussen said.