Russian forces have advanced 1 kilometer (about 0.6 miles) into the northeastern Kharkiv region of Ukraine near Vovchansk, according to a high-ranking Ukrainian military source.

The source said the Russian military was aiming to advance as much as 10 kilometers into the region on Friday in an effort to establish a buffer zone. The source told the Reuters news agency that Ukrainian forces were fighting to hold back Moscow's advance.

The Ukrainian Defense Ministry earlier said Russian forces tried to break through the border into Kharkiv, employing armored vehicles and heavy shelling in what is being seen as a possible bid to open a new front in its war on its neighbor.

"At approximately 5 a.m., there was an attempt by the enemy to break through our defensive line under the cover of armored vehicles," the ministry said.

Kyiv says it has sent reinforcements, but fighting is still raging in the border area.

Russia has also stepped up shelling the region. A local official in the town of Vovchansk said evacuations were ongoing due to intensified Russian shelling.

The reported incursion comes amid a recent buildup of Russian troops near the Kharkiv region, which borders Russia, that could herald an impending attack away from Moscow's previous focus on the Donetsk area.