The White House said on Wednesday that Russia has failed to take steps needed to ease the crisis in Ukraine, striking an ominous tone that appeared to position the United States for a new round of economic sanctions against Moscow.

President Barack Obama in recent weeks has repeatedly threatened fresh new sanctions, and appears now to have run out of patience as fighting continued to rage in Eastern Ukraine.

"There are some clear steps that we've asked Russia to take that they haven't taken. And that is what has elevated the risk that Russia faces right now as it relates to additional economic costs that could be imposed by the international community," White House spokesman Josh Earnest told reporters.

Any new sanctions would likely be broader than the targeted penalties the United States has already imposed against specific Russian individuals and businesses following Russia's incursion in Ukraine and its March 18 annexation of the Crimea region.

Obama, who met Secretary of State John Kerry at the White House on Wednesday, discussed Ukraine in a phone call on Tuesday with German Chancellor Angela Merkel.

Vice President Joe Biden had a phone conversation on Tuesday with Ukrainian President Petro Poroshenko that appeared to set the stage for sanctions.

"The vice president told President Poroshenko that the United States was engaging with European leaders to discuss the imposition of costs on Russia for its continued escalation of the conflict," a White House statement said.

European Union leaders meeting in Brussels also discussed a fresh round of sanctions against Russia. A draft statement seen by Reuters showed they want to block loans for new projects in Russia by two multilateral lenders and to broaden sanctions to target companies that help destabilize Ukraine.

Washington said on Wednesday up to 12,000 Russian forces are back on the border with Ukraine and that weaponry was crossing over to pro-Russian separatists.

"These are combat forces," Pentagon spokesman Colonel Steve Warren told reporters.

The increase in the Russian presence occurred several weeks after Moscow had drawn down its forces in the area to about 1,000 troops.

Warren said the size of the force there now would allow Russia to conduct operations on either side of the border.

"While I can't speak for what they intend to do. It is intimidating," he said.