New Russian Sukhoi-34 twin-seat fighter jets arrived at an air base in southern Russia on as a cease-fire agreement with Ukraine collapsed amid accusations that Ukraine violated it. (Reuters)

New Russian Sukhoi-34 twin-seat fighter jets arrived at an air base in southern Russia on as a cease-fire agreement with Ukraine collapsed amid accusations that Ukraine violated it. (Reuters)

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.

1 of 22 Full Screen Autoplay Close June 13, 2014 Skip Ad × Ukraine retakes Mariupol, part of border View Photos Ukrainian forces ousted pro-Russian separatists from a port city in the east and regained control of a stretch of the border with Russia on Friday, the government in Kiev announced. The attack on separatist strongholds in Mariupol, the second-largest city in the Donetsk region, was over in less than six hours, an Interior official said. Caption Ukrainian forces ousted pro-Russian separatists from a port city in the east and regained control of a stretch of the border with Russia on Friday, the government in Kiev announced. The attack on separatist strongholds in Mariupol, the second-largest city in the Donetsk region, was over in less than six hours, an Interior official said. June 14, 2014 Pro-Russian fighters secure the area around a downed Ukrainian military transport plane in the eastern city of Luhansk, Ukraine. The Ukrainian prosecutor's office has confirmed that 49 people were killed after separatists shot down the Ilyushin-76 air force plane as it was approaching Luhansk’s airport. Mstyslav Chernov/European Pressphot Agency Buy Photo Wait 1 second to continue.

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.

Fighting between separatists and government troops continued at daybreak Friday morning, when rebels opened fire on the Luhansk airport, according to the Ukrainian Interfax news agency. The airport is in the middle of a rebel stronghold, but is being held by Ukrainian army troops. Last weekend, militants shot down a military transport plane attempting to land there.

On Thursday, 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 300 insurgents and seven Ukrainian soldiers had been killed.

Igor Strelkov, a rebel leader in Slovyansk, went on YouTube and sardonically berated Moscow over what he implied was insufficient assistance. Standing before a pile of sandbags, he said Ukrainian troops had overwhelmed separatists in the nearby villages of Yampil and Servisk. He said that the rebels would continue fighting but that the Ukrainian military would break their defenses soon.

“I am grateful to the Russian Federation, which supports us so strongly,” he said sarcastically, adding: “And I still hope Moscow has conscience enough to take action.”

In a separate YouTube video posted Thursday night, one of several he sent throughout the day, Strelkov’s tone grew more urgent as he pleaded with Moscow for help.

“The militia and the People’s Republic of Donetsk urge the Russian leadership to provide urgent assistance to New Russia to prevent further genocide of the Russian population,” he said.

Near the town of Krasny Lyman, the Ukrainian army distributed fliers asking the militants to surrender their arms. It repeated an amnesty offer made by Poroshenko to those who were not directly responsible for bloodshed and promised a fair investigation into their actions to determine whether they are eligible.

One ray of hope came from the Organization for Security and Cooperation in Europe, which announced that it had been in contact with two teams of monitors that have been missing for three weeks and were last seen at checkpoints in eastern Ukraine. They are unharmed, OSCE said.

Meanwhile, Ukraine’s richest man, billionaire Rinat Akhmetov, issued a plea for peace using short, staccato sentences:

“People do not need a process. People need a result. People need peace!

“They are asking, they are demanding — please give us peace.

“I strongly believe that we must find a peaceful solution.

“And we will find it only at the negotiating table.

“Not at the wheel of armored vehicles, or by machine guns and artillery.”

Michael Birnbaum in Chisinau, Moldova, contributed to this report.