Vice President Biden spoke on the phone with Ukrainian President Petro Poroshenko on Friday after a rebel offensive this week threatened increased violence between Russia and Ukraine.

Biden strongly condemned the Russian-backed offensive earlier this week in which rebels seized the eastern Ukrainian town of Debaltseve, according to a readout of his call provided by the White House. The U.S. has said the offensive violated multiple cease-fire agreements signed by Russia.

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In his call with Poroshenko and a separate talk with Ukrainian Prime Minister Arseniy Yatsenyuk, Biden discussed how the cease-fire and withdrawal of heavy weaponry could be monitored. 

What remains unclear is whether the U.S. will provide lethal weapons to Ukraine or decide to ramp up economic sanctions against Russia. 

A bipartisan group of lawmakers wrote a letter to Secretary of State John KerryJohn Forbes KerryOvernight Energy: Farmers say EPA reneged on ethanol deal | EPA scrubs senators' quotes from controversial ethanol announcement | Perry unsure if he'll comply with subpoena | John Kerry criticizes lack of climate talk at debate John Kerry calls out lack of climate questions at debate Democrats' debate divisions open the race to new (or old) faces MORE on Friday urging the U.S. provide defensive arms to Ukraine and explore the possibility of additional sanctions on Moscow. 

White House press secretary Josh Earnest on Friday said "it's possible that there could be additional costs over and above" current sanctions if Russia not abide by its agreements. 

The Ukrainian government says that Russian-backed rebels have violated the cease-fire more than 250 times since it officially took effect Feb. 15, State Department spokeswoman Jen Psaki said Thursday. 

Germany, France, Russia and Ukraine have said that last week's truce must be enforced despite the rebel offensive in Debaltseve, according to Bloomberg