Vladimir Isachenkov and John-Thor Dahlburg

Associated Press

MOSCOW (AP) — Intense fighting raged Thursday in eastern Ukraine, where pro-Russia insurgents said they were losing the battle with government forces and begged the Kremlin for military help.

NATO's chief, meanwhile, accused Russia of resuming a military buildup along the border designed to intimidate Ukraine.

Russian President Vladimir Putin has been under pressure from nationalists at home who have demanded that he send troops into eastern Ukraine to help the rebels. But Putin has stonewalled insurgents' pleas to join Russia and welcomed a peace plan and proposed cease-fire put forward by new Ukrainian President Petro Poroshenko.

That plan, however, requires a secure border between Russia and Ukraine, Poroshenko says — and it's not clear when that will happen.

Both the Ukrainian government and the rebels said heavy fighting took place Thursday near Krasnyi Liman, just east of the rebel stronghold of Slovyansk, a city in the Donetsk region that has been the epicenter of the insurgency against the government in Kiev.

Vladislav Seleznev, a spokesman for Ukrainian forces in the east, said in a statement posted on Facebook that four government troops were killed and 20 wounded in fighting Thursday. He said up to 200 rebels were killed and hundreds more were wounded in the battle. That statement could not be independently confirmed.

Rebel chief Igor Strelkov, however, said in a statement on YouTube that his men were far outnumbered and outgunned by Ukrainian forces and were likely to retreat from their positions in Yampol and Seversk near Krasnyi Liman. He said the Ukrainian military advance would completely cut rebel supply lines to Slovyansk and issued a desperate plea to the Kremlin for military assistance.

"I hope that they have enough conscience left in Moscow to take some measures," Strelkov said.

Russia has dismissed Ukrainian and Western claims that it was fomenting the rebellion in the east, saying that Russian citizens among the rebels are volunteers and rejecting Western reports that it has sent heavy artillery across the border to the rebels.

Denis Pushilin, one of the insurgent leaders in Donetsk, told reporters Thursday in Moscow that the rebels did have a few tanks but insisted they had seized them from Ukrainian forces. The U.S. government, however, has said it had evidence that tanks and heavy military equipment came over the border from Russia to Ukraine.

As pitched battles continued in Ukraine, NATO Secretary General Anders Fogh Rasmussen said Thursday that Russia had resumed a military buildup near the Ukrainian border, calling it "a very regrettable step backward."

"I can confirm that we now see a new Russian military buildup — at least a few thousand more Russian troops deployed to the Ukrainian border — and we see troop maneuvers in the neighborhood of Ukraine," Fogh Rasmussen said in London. "If they're deployed to seal the border and stop the flow of weapons and fighters that would be a positive step. But that's not what we're seeing."

The Russian Defense Ministry refused to comment on the NATO chief's statement.

Russian officials responded angrily to previous NATO claims of a massive Russian military presence near the 2,000-kilometer border, calling them overblown and insisting that the troops there were stationed far from the border and were involved in regular training.

Still, Putin last month ordered troops near the border to return to their permanent bases elsewhere in Russia, casting it as a move to assuage tensions. NATO said in late May that the bulk of an estimated 40,000 troops had pulled back.

The new Russian military deployments come at a delicate time. On Wednesday, Ukraine's new president promised a unilateral cease-fire to jump start his plan to end fighting in the country's east.

Next week, the foreign ministers and leaders of the European Union are meeting and relations with Ukraine and Russia will be key topics. The leaders are expected to consider whether Russia's actions toward Ukraine warrant imposing tougher economic sanctions.

Rasmussen said Russia appears to be using its military to intimidate Ukraine.

"I consider this a very regrettable step backward and it seems that Russia keeps the option to intervene further," Rasmussen said. "The international community would have to respond firmly if Russia were to intervene further. That would imply deeper sanctions, which would have a negative impact on Russia."

In his speech at Chatham House, Rasmussen said the U.S.-led NATO alliance is at a turning point.

"The world that we helped to build after the end of the Cold War is being challenged in different ways and from different directions," he said.

"To our east, Russia's aggression against Ukraine is an attempt to rewrite international rules and recreate a sphere of influence. At the same time, to our south, we see states or extreme groups using violence to assert their power. And overall, we see threats old and new, from piracy to terrorism to cyberattacks."

Dahlburg reported from Brussels.