(Adds Hagel comments)
BRUSSELS, Feb 27 (Reuters) - Russia must be transparent about military exercises along Ukraine's border and not take any steps that could be misinterpreted or "lead to miscalculation during a delicate time," U.S. Defense Secretary Chuck Hagel said on Thursday.
Russian President Vladimir Putin ordered 150,000 troops to be ready for war games near Ukraine on Wednesday, and on Thursday, Russia put fighter jets on combat alert.
Hagel, following NATO talks on Ukraine, said the United States expected other nations to respect Ukraine's sovereignty and avoid provocative actions.
"That's why I'm closely watching Russia's military exercises along the Ukrainian border," Hagel said at a NATO news conference. "I expect Russia to be transparent about these activities, and I urge them not to take any steps that could be misinterpreted, or lead to miscalculation during a very delicate time -- a time of great tension."
Hagel said he hoped to speak with his Russian counterpart within the next day or two.
On Wednesday, U.S. Secretary of State John Kerry warned Russia it would be a "grave mistake" to intervene militarily in Ukraine and said it was considering $1 billion in U.S. loan guarantees and additional funding to help Kiev.
Earlier this month, a Kremlin aide warned that Moscow could intervene and accused Washington of breaching their 1994 treaty, under which Russia removed Soviet nuclear weapons from Ukraine.
It is unlikely the United States and its allies in NATO would risk an outright military confrontation with Russia. But the Russian rhetoric, ringing with echoes of the Cold War, underlines the high stakes in Ukraine, whose 46 million people and sprawling territory are caught in a geopolitical tug-of-war.
Armed men seized the regional government headquarters and parliament in Ukraine's Crimea on Thursday and raised the Russian flag. The takeover alarmed Kiev's new rulers, who urged Moscow not to abuse its navy base rights on the peninsula by moving troops around.
Asked about the situation in Crimea, Hagel said the United States was following the situation closely. He noted the lack of detail and said it was important to "keep the tensions down."
"This is a time for really cool, wise leadership, on the Russians' side, and on everyone's side," he said. (Reporting by Phil Stewart; Editing by Larry King)