Ukraine and its Western allies say Russia has started a "full-scale invasion" of the Eastern European nation.
Russian forces, backed by pro-Russian separatists, have captured the key Ukrainian port town of Novoazovsk.
"It is an absolutely clear-cut case of a larger country bullying a smaller country," Australian Prime Minister Tony Abbott said.
Samantha Power, U.S. ambassador to the United Nations, said, "In the last 48 hours Russia's actions have spoken volumes."
CBN Ministries are at work in Ukraine in the midst of the fighting. Reporter Heather Sells spoke with Terry Meeuwsen of Orphan's Promise about the situation on the ground.
NATO said satellite images show more than 1,000 Russian combat troops are now operating inside south eastern Ukraine. But that's only the tip of the iceberg.
There are claims that as many as 15,000 Russian soldiers have been sent to the Donetsk and Luhansk regions.
"We have also detected large quantities of advanced weapons, including air defense systems, artillery, tanks, and armored personnel carriers being transferred to separatists forces in eastern Ukraine," Brig. Gen. Nico Tak, director of NATO's Comprehensive Crisis Operations Management Center, said.
This new front in the southeast opens the prospect that separatists are trying to create a land link between Russia and Crimea. If this happens, the Kremlin will control the Sea of Azov, which has significant gas and mineral riches.
Ukraine's national security council has ordered mandatory conscription to counter what the country's president calls an extremely difficult threat.
"We are capable of defending ourselves. The main thing is not to panic," a Ukrainian presidential spokesperson tweeted.
Russia's latest move is another sign that economic sanctions have failed to stop President Vladimir Putin's land grab.
"Putin and his inner circle are willing to take on a lot of economic damage. They see this as the kind of fight they are willing to go at it," Boris Zilberman, with the Foundation for Defense of Democracies, said.
Nevertheless, President Barack Obama avoided calling Putin's actions an "invasion" and said a military confrontation with Russia wasn't in the cards.
"We are not taking military action to solve the Ukrainian problem," the president said. "What we're doing is to mobilize the international community to apply pressure on Russia."
Analysts warn Russia's next target could be the strategic port city of Mariupol. Ukraine has reinforcements in place to stop any potential opening of a land corridor to Crimea.