Washington (AFP) - The United States gave a clear signal Wednesday that Russia will face further sanctions for what the White House says is Moscow's role in fueling violence in eastern Ukraine.

Following a telephone call between US Vice President Joe Biden and Ukrainian President Petro Poroshenko, the White House condemned "Russian-backed separatists" and the "heavy toll that the Russian-backed offensive in the east was having on Ukraine's civilian population."

"As long as Russia continues its blatant disregard of its obligations... the costs for Russia will continue to rise," Biden was reported to have told the Ukrainian leader.

Late Tuesday President Barack Obama and German Chancellor Angela Merkel discussed the "significant increase" in violence in Ukraine and warned Russia would be held accountable.

"The two leaders expressed concern about the significant increase in violence in eastern Ukraine and Russia's material support for the separatists," a White House statement said.

"They agreed on the need to hold Russia accountable for its actions."

Western sanctions and a slide in oil prices have plunged Russia into recession and seen Standard and Poor's slap a "junk" rating on Moscow's foreign currency debt.

Pro-Russian rebels in Ukraine last week defiantly pulled out of peace talks and promised an offensive on a strategic government-held port city that provides a direct land bridge to Ukraine's Russian-occupied Crimea peninsula.

Russia denies backing the eastern insurgents and says that measures against it are designed to undermine President Vladimir Putin's 15-year rule.

Obama has issued an executive order prohibiting trade with Crimea, the Black Sea peninsula that Moscow annexed from Ukraine in March.

Two dozen people -- including Russians and separatists -- have also been added to a US blacklist, subjecting them to travel bans and assets freezes.