Ukrainian President Petro Poroshenko on Saturday told U.S. Vice President Joseph Biden that the separatists were using heavy weapons on the government forces in their counterattacks aiming to retake lost territories in Ukraine's east.

In their phone conversation, Poroshenko informed the veep " about the latest separatist attacks using heavy weapons on Ukrainian forces and the vice president expressed his condolences for the loss of life," the White House said in a statement.

It said Biden voiced support for the president's efforts to convene a meeting to discuss a possible ceasefire with the separatists, and talked about ongoing U.S. diplomatic efforts to impose additional costs on Russia "if it continues on its current course of providing the separatists with heavy weapons and equipment."

The Ukrainian military said on Saturday the insurgents had started to launch counterattacks trying to recapture cities lost to the government in recent days, while government troops were continuing their offensive.

Poroshenko ended a 10-day ceasefire on July 1 in eastern Ukraine, blaming the insurgent forces for failing to observe the truce. The Ukrainian military then resumed its offensive in the states of Donetsk and Lugansk, and have since seized two thirds of territories in the two states.

Russia has dismissed as "shameless" the West's accusations that it is supplying the Ukrainian rebels with arms.