Ukrainian forces targeted positions near the border with Russia from where separatists, using high-powered Grad missiles, bombarded an army motorised brigade on Friday, killing 23 servicemen.
Friday's official death toll was the highest since Poroshenko tore up a brief ceasefire on 1 July and relaunched an offensive that managed to dislodge the rebels from key eastern strongholds they have controlled for nearly three months.
Biden, who has been the point man for the White House on the Ukrainian crisis, spoke to Poroshenko by phone earlier in the day.
"President Poroshenko informed the vice-president 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.
"[Biden] expressed support for President Poroshenko's efforts to convene a meeting to discuss a possible ceasefire with the separatists, and informed Poroshenko of ongoing US diplomatic efforts to work with our international partners to impose costs on Russia if it continues on its current course of providing the separatists with heavy weapons and equipment."
The US has warned repeatedly that it will impose further sanctions, along with its western allies, against Russia if Moscow did not use its influence to tame the separatists and make peace with Kiev.
The European Union on Saturday imposed new sanctions on leading rebel figures.
Source: The Guardian Today