Ukraine on Sunday placed its armed forces on combat alert, closed its airspace to non-commercial flights and partially mobilized its reservists in the face of the growing Russian military intervention in the autonomous republic of Crimea, which is populated mainly by ethnic Russians.

The country's interim president, Olexandr Turchinov, said that "Russian troops are blocking Ukrainian military units," adding that Kiev will consider any offensive moves against those units to be an attack by Russia.

Turchinov said that the Russian military command issued an ultimatum to Ukrainian forces to turn over their weapons and abandon their units before 5 a.m. (0300 GMT) on Sunday.

"Although the ultimatum was not obeyed, there was no attack. The situation continues to be tense," Turchinov added.

On Saturday, Russian President Vladimir Putin requested and obtained from his country's parliament authorization to use Russian armed forces throughout Ukrainian national territory.

Meanwhile, in Kiev thousands of people gathered on Sunday in central Independence Square to demonstrate against war and show their support for the new Ukrainian government.

In other developments, NATO called on Russia to respect Ukrainian territorial integrity, adhere to the U.N. charter, work to reduce regional tensions and withdraw its troops from that country.

Authorities in Crimea, however, openly rebelled against Kiev's authority by creating their autonomous republic's own navy and expressing their intention to acquire state status via a referendum on March 30. They also said that they recognized the authority of Ukraine's "legitimate president," Viktor Yanukovych, who was ousted and fled to Moscow last week.

Also, Putin agreed to a proposal by German Chancellor Angela Merkel made in a telephone conversation Sunday afternoon to allow a fact-finding mission to come to Ukraine and to begin a political dialogue.

U.S. Secretary of State John Kerry will travel to Kiev on Tuesday to express U.S. support to authorities in Kiev. Kerry said on Sunday that Russia is running a serious risk of losing its international position as a member of the G8 if it does not withdraw from Ukraine.

Crimea is formally a part of Ukraine, but Russian is far more widely spoken than Ukrainian and two-thirds of its population are ethnic Russians. It is also where Moscow bases its Black Sea fleet. EFE