Troops displaying no identifying insignia but were mingling with local pro-Russian militants stand guard outside a government building on March 2, 2014, in Simferopol, Ukraine.

NATO leaders will respond to the Ukraine crisis by agreeing this week to create a "spearhead" rapid reaction force, potentially including several thousand troops, that could be sent to a hot spot in as little as two days, officials said on Monday.

The 28-nation alliance already has a rapid reaction force but U.S. President Barack Obama and other leaders meeting for a NATO summit in Wales on Thursday and Friday are expected to create a new force that would be able to respond more quickly to a crisis.