PEREVALNE, Ukraine -- Pro-Russian volunteer militias on Monday bolstered the ranks of Russian troops who surrounded a Ukrainian base in Crimea over the weekend.
On Sunday, Russian troops demanded the Ukrainian soldiers inside the base lay down their weapons in a tense standoff as the security crisis in the region deepened.
The Ukrainian forces in Perevalne -- also known as Privolno -- remained defiant, but were heavily outnumbered.
Dozens of members of a pro-Russian "self defense" force created a cordon around the site on Monday. The groups say they are helping to keep the peace in cities and at checkpoints and bases.
Ukrainian forces have been trapped inside their bases throughout the semi-autonomous peninsula that was seized by Russia.
By 7:10 a.m. ET on Monday, no shots had been fired in what appeared to be the first direct confrontation between the Russian and Ukrainian military.
Ukraine mobilized for war on Sunday after Russian President Vladimir Putin declared he had the right to invade his neighbor to protect Russian citizens.
The standoff in Ukraine has created the greatest moment of tension between Russia and the West since the collapse of the Soviet Union in 1991, an event Russian President Vladimir Putin once called the worst geopolitical catastrophe of the 20th century.
Ed Flanagan and Bill Neely of NBC News and Reuters contributed to this report.