Pro-Moscow rebel forces in eastern Ukraine, backed by Russian tanks and armored personnel carriers, battled government forces on two fronts Thursday, a Ukrainian military official said.

The fighting was taking place southeast of Donetsk, and along the nation's southern coast in the town of Novoazovsk, about 12 miles (20 km) from the Russian border, according to Mykhailo Lysenko, the deputy commander of the Ukrainian Donbas battalion.

"This is a full-scale invasion," Lysenko said, referring to the fighting in the south.

In a Cabinet meeting Thursday, Ukraine's Prime Minister, Arseniy Yatsenyuk, called for an immediate U.N. Security Council meeting.

"Russia has very much increased its military presence in Ukraine," he said.

The latest flareup comes despite a meeting between Ukrainian President Petro Poroshenko and Russia's Vladimir Putin in Minsk on Tuesday at which some progress appeared to have been made toward finding a diplomatic solution to the crisis.

U.S. Ambassador to Ukraine Geoffrey Pyatt also said Thursday that Russian soldiers were directly involved in the fighting, alongside the pro-Russia rebels.

"Russian supplied tanks, armored vehicles, artillery and multiple rocket launchers have been insufficient to defeat Ukraine's armed forces, so now an increasing number of Russian troops are intervening directly in the fighting on Ukrainian territory," he said on Twitter.

"Russia has also sent its newest air defense systems including the SA-22 into eastern Ukraine and is now directly involved in the fighting."

Moscow denies supporting and arming the pro-Russia rebels. It has also repeatedly denied allegations by Kiev that it has sent troops over the border.

Yatsenyuk suggested tougher measures may be needed to curb Russia's support for the rebels.

"Unfortunately, the sanctions were unhelpful as to deescalating the situation in Ukraine," he said, referring to the economic sanctions already imposed by the United States and European Union against Russian individuals and companies.

Yatsenyuk suggested one way to halt "Russian aggression" could be to freeze all assets and ban all Russian bank transactions until Russia "pulls out all its military, equipment and agents" from Ukraine.

"Vladimir Putin has purposely started a war in Europe. It is impossible to hide from the fact," he said.

(CNN)