DONETSK, Ukraine -- Signs emerged Friday pointing to a major offensive in eastern Ukraine by rebels fighting to break off from the rest of the country.

A rebel leader said his forces would continue to fight and wouldn't join further peace talks, as Russian President Vladimir Putin blamed the deteriorating security situation squarely on Ukrainian leaders in Kiev.

NATO said it has indications that a major rebel offensive is coming, due to heavy weapons moving in from Russia as they have prior to previous rebel pushes. Meanwhile, the U.N. human rights agency said the overall death toll in the conflict had risen above 5,000 as fighting escalates.

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Separatist leader Alexander Zakharchenko said rebel fighters would continue to fight for more territory, and were advancing in five directions to push back Ukrainian government forces to the limits of the eastern Donetsk region.

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

Any rebel advances would further undermine a tentative peace deal forged this week in Berlin at a meeting of foreign ministers from Russia, Ukraine, France and Germany. Those negotiations concluded with an agreement to uphold a demarcation line defined in September after talks in the Belarusian capital, Minsk.

The plan calls for Ukrainian troops and Russian-backed separatists to pull back their heavy arms 9 miles on either side of the line, though there was no agreement on a withdrawal of troops.

According to the Reuters news agency, Putin said in Moscow that officials in Kiev "have given an official order to start large-scale military operations practically throughout the whole line of contact. The result is tens of killed and wounded, not only among the military on both sides but... among civilians."

"Those who give these criminal orders are responsible," he told state officials in televised remarks.

The urgency for a fresh settlement peaked over the weekend as battles intensified over control over the airport on the northern edge of the separatist stronghold of Donetsk. Clashes culminated with rebels taking control over the terminal, although fighting is continuing in nearby settlements.

Separatist officials said Ukrainian forces were still targeting the airport.

Ukrainian President Petro Poroshenko appeared Thursday to still be holding out hope for a new cease-fire, but said stern retribution would await anybody violating the peace.

"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 town, we will smash them in the teeth," Poroshenko told a meeting of top defense officials.