Russian forces appear to be tightening their grip on the peninsula. Ukrainian troops mobilize in response.

Defying calls from Western powers to deescalate its intervention in the Crimea, Russia appeared to be expanding its military presence there on Saturday when Ukrainian army units were also mobilized across the country.

A massive column of Russian armored personnel carriers, trucks and support vehicles was filmed on the road from Krasnodar to Novorossiysk, on the Black Sea coast, just east of the Crimea, apparently preparing to support the invasion.

An Associated Press reporter trailed another convey in the Crimea itself on Saturday afternoon that pulled into a military airfield north of Simferopol, the regional capital.

An Ukrainian patrol plane came under fire while flying close to the Crimea’s administrative border, guards said.

Novaya Gazeta , an independent Russian newspaper, reported that Russian troops had established a minefield on a small peninsula northeast of the Crimea that provides one of only two access routes into the area. The smaller peninsula is part of Ukraine’s Kherson Oblast.

An Ukrainian army spokesman told AP that witnesses had reported seeing amphibious ships unloading around two hundred military vehicles in eastern Crimea on Friday night after apparently having crossed the Strait of Kerch that separates the Crimea from Russia’s territory.

“Neither the equipment nor the paratroopers have insignia that identify them as Russian but we have no doubt as to their allegiance,” the spokesman said.

Russian president Vladimir Putin denied earlier this week that he had sent soldiers to the Crimea but vowed to protect Russian speakers and Russian interests in a region that was part of the Russian Empire for almost two centuries before Soviet leader Nikita Khrushchev transferred it to Ukraine in 1954.

The troops, which Putin suggested belonged to “local self-defense forces,” were nevertheless wearing Russian uniforms and driving vehicles with Russian license plates. The vehicles spotted north of Simferopol on Saturday were without plates.

The new government in Kiev — which took power in late February after the country’s president, Viktor Yanukovich, fled to Russia in the face of mass protests against his decision to pull out of trade talks with the European Union — has shied away from challenging Russia’s intervention in the Crimea but Prime Minister Arseniy Yatseniuk warned on Thursday that its military would react if Russia pushed deeper into Ukrainian territory.

Ukrainian army units appeared to be mobilizing for a possible response on Saturday. A YouTube video purported to show tanks being loaded on railway carriages in Bila Tserkva, a small city south of Kiev. Another video showed armored personnel carriers leaving their base near Lviv in the west of Ukraine. The Ministry of Defense said those forces were deployed on a training mission.

A third video showed Buk missile systems being deployed in the Donetsk Oblast in the east of the country, a region with a 38 percent ethnic Russian population. These systems can be used to shoot down cruise missiles as well as aircraft.

Russia’s armed forces far outnumber those of Ukraine. It has about four times as many soldiers, twice as many tanks and six times as many combat aircraft. However, only part of those forces can be deployed in Ukraine, a Radio Free Europe/Radio Liberty analysis pointed out. Russia cannot afford to remove forces from its North Caucasus region, from its border with China or from the Pacific.