DNIPROPETROVSK, Ukraine — As government troops and insurgents were locked in heavy battles in eastern Ukraine on Thursday, NATO’s chief said a new Russian military buildup was underway near the border.

A rebel leader took to YouTube to say that pro-Russian fighters were being overrun by Ukrainian troops, and he alternately taunted and cajoled Moscow to rush to their aid. The army blanketed the countryside with fliers urging rebels to surrender their arms in exchange for amnesty, and it claimed that it had inflicted hundreds of casualties on the insurgents.

The rapid-fire turn of events unfolded just two weeks after Petro Poroshenko was sworn in as Ukraine’s president and suggested that more blood may be shed before there is any hope for peace.

The reports of a Russian army buildup at the very least indicate that the border is growing increasingly militarized, a sign of heightened tensions between the two neighboring states.

In what NATO Secretary General Anders Fogh Rasmussen called “a very regrettable step backwards,” he estimated that Russia has redeployed “at least a few thousand” troops near the border with Ukraine.

Ukrainian defense officials have noted the buildup in recent days, and the National Security and Defense Council said Thursday that Russian military personnel were observed throwing camouflage nets over equipment already in place even as the army moved new troops forward.

The Ukrainian statement identified four Russian units that it said were at the border – two airborne divisions, an airborne assault brigade and a motorized rifle brigade. It also said that a military convoy stretching almost 10 miles was spotted on the road between Moscow and the Ukrainian capital, Kiev. The convoy was not moving, just parked on the roadside.

The reports could not be independently verified.

Meanwhile, fighting raged in the countryside of the Donetsk region, one of two separatist strongholds in eastern Ukraine. The Defense Ministry said the army had regained control of two villages, Yampil and Kirovsk.

Vladislav Seleznev, a spokesman for Ukrainian troops in the east, said 200 insurgents and four Ukrainian soldiers had been killed.

Near the town of Krasny Lyman, the Ukrainian army distributed fliers asking the militants to surrender their arms.