WASHINGTON - US Vice President Joe Biden warned Russia over Ukraine after new columns of tanks, trucks and heavy artillery rumbled towards the pro-Moscow rebel stronghold of Donetsk on Monday.
The West has repeatedly expressed concern at Ukrainian claims that Russian military reinforcements are being sent in, while Moscow denies that it is involved in the fighting in the eastern part of its neighbour.
Biden spoke to Ukrainian President Petro Poroshenko by telephone, with the pair agreeing it was "critical" for Russia to honour its commitments to a ceasefire agreement signed in September in the Belarussian capital Minsk.
"The vice president noted that if Russia continued to wilfully violate the terms of the Minsk agreement, the costs to Russia will increase," a White House statement said.
It added: "The vice president urged the speedy conclusion of a coalition agreement to enable the Ukrainian government to continue the process of passing and then implementing reforms, and delivering on other commitments made to the Ukrainian people in the October 26 parliamentary elections."
The conflict in Ukraine has sent relations between Western backers of Ukraine and Russia to their lowest levels since the end of the Cold War.