DONETSK, Ukraine -- Pro-Russian rebels in eastern Ukraine rejected a previously signed peace deal Friday and launched a new multipronged offensive against Ukrainian government troops, upending recent European attempts to mediate an end to the fighting.

The main separatist leader in the rebellious Donetsk region vowed to push Ukrainian soldiers out of the area and said insurgents would not take part in any more cease-fire talks. Another rebel went even further, saying they would not abide by a peace deal signed in September.

Separatist leader Alexander Zakharchenko said rebel fighters went on the offensive to gain more territory and forestall a Ukrainian attack. He declared they would push government troops to the border of the Donetsk region and possibly beyond.

"Attempts to talk about a cease-fire will no longer be undertaken by our side," Zakharchenko said.

The peace deal signed in September in the Belarusian capital of Minsk envisaged a cease-fire and a pullout of heavy weapons from a division line in eastern Ukraine. It has been repeatedly violated by both sides, and heavy artillery and rocket barrages have increased the civilian death toll in the last few weeks.

Foreign ministers from Russia, Ukraine, France and Germany agreed Wednesday to revive that division line, but fighting has continued unabated. The U.N. human rights agency on Friday raised its estimate of the conflict's overall death toll to nearly 5,100 since April.

The tentative peace deal forged this week in Berlin called for Ukrainian troops and Russian-backed separatists to pull back their heavy arms 15 kilometers (9 miles) on either side of the line, although there was no agreement on a withdrawal of troops.

But rebel spokesman Eduard Basurin threw that agreement into doubt, saying the insurgents "will no longer consider the Minsk agreement in the form it was signed," although he added that they will remain open for peace talks.

Basurin's bold statement contradicted the official position of Russia, which has repeatedly pledged respect for the Minsk agreement, even though it has been reluctant to meet its end of the deal, which also requested the withdrawal of foreign fighters and the monitoring of the Russian-Ukrainian border by the Organization for Security and Cooperation in Europe.

Battles intensified last weekend over Donetsk airport, a gleaming showcase for the Euro 2012 soccer championship that has been reduced to rubble by months of clashes. Rebels eventually took control of its terminal. Fighting has continued on its fringes.

Zakharchenko said rebel fighters were advancing in three directions in the Donetsk region and also pressing their attack in two other areas in the Luhansk region.

"We will hit them until we reach the border of Donetsk region, and ... if I see the danger for Donetsk from any other city, I will destroy this threat there," he said.

A top NATO official confirmed that rebels had pushed west and received reinforcements. U.S. Air Force Gen. Philip Breedlove said air-defense and electronic-warfare equipment have been detected in eastern Ukraine -- hardware that, in the past, coincided with the incursion of Russian troops into Ukraine.

A pro-Russian insurgency flared up in April in the Donetsk and Luhansk regions in eastern Ukraine following Russia's annexation of Ukraine's Crimean Peninsula. Russia insists that it does not support the rebels, but Western military officials say the sheer number of heavy weapons under rebel control belies that claim.

At the international economic forum in Davos, Switzerland, a Russian deputy prime minister vowed that Moscow would not be cowed by the sanctions the West has imposed upon Russia for its actions in Ukraine.

Igor Shuvalov warned the West against trying to topple Russian President Vladimir Putin, reflecting the Kremlin's view that the European Union and U.S. sanctions are aimed at regime change.

"When a Russian feels any foreign pressure, he will never give up his leader," Shuvalov said Friday. "We will survive any hardship in the country -- eat less food, use less electricity."

The Russian currency has lost half its value in recent months from the double blow of sanctions and a plunge in world oil prices.

Ukrainian President Petro Poroshenko said stern retribution would await anybody violating the peace. After a speech Wednesday at Davos, he rushed home to deal with the escalating fighting.

"If the enemy doesn't want to abide by the cease-fire, if he doesn't want to put an end to the suffering of peaceful people, Ukrainian villages and towns, we will smash them in the teeth," Poroshenko told top defense officials.

In Washington, U.S. State Department spokeswoman Jen Psaki voiced concern about the increasing bloodshed.

"Ukraine has implemented cease-fire after cease-fire, but the Russia-backed separatists have responded with violence," she said, citing 1,000 attacks since early December and the deaths of 262 people in the last nine days.

Russia, she added, "holds the keys to peacefully resolving a conflict it started and bears a responsibility to end the violence."