Russia’s Defense Ministry announced Thursday morning that warplanes will be put on alert on the Ukrainian border, Channel 10 reported.

The announcement followed orders Wednesday for 150,000 Russian troops to go on high alert. Troops are planning to hold war games near Ukraine, in what has widely been seen as a warning gesture from Russia over the ouster of Ukrainian President Victor Yanukovich, a Russian ally.

Also Wednesday, militants seized control of a central government building in the Crimea Peninsula, which is under Ukrainian sovereignty, and hoisted a Russian flag.

In 2008, Russia invaded Georgia, with the stated goal of protecting two districts that had proclaimed independence, and that were home to many ethnic Russians and Russian citizens. In recent days, Russian leaders have expressed concern for the safety of Russian citizens in Ukraine.

Acting Ukraine President Olexander Turchinov warned Moscow against crossing the lines set in previous agreements between the two countries. Any move across the border or “unusual troop movement” will be considered a military invasion, he warned.

United States Secretary of State John Kerry has joined Ukrainian protest leaders in warning Russia against intervention.

“Any kind  of military intervention that would violate the sovereign territorial integrity of Ukraine would be a huge – a grave mistake,” Kerry declared Thursday.

Tumult in Kiev led to the pro-Russian Yanukovich being toppled and replaced last weekend. Earlier this week Russia recalled its ambassador.