U.S. official says 1,000 Russian troops enter Ukraine

Kiev, Ukraine (CNN) -- A top Ukrainian army officer said a "full-scale invasion" of his country was under way Thursday, as a U.S. official said up to 1,000 Russian troops had crossed Ukraine's southern border to fight alongside pro-Russian rebels.

U.S. officials said Russian troops were directly involved in the latest fighting, despite Moscow's denials.

Rebels backed by Russian tanks and armored personnel carriers fought Ukrainian forces on two fronts Thursday: southeast of rebel-held 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.

More