More pressure on Russia

"The President underscored the need for the United States, European Union, and other global partners to be prepared to meet further Russian escalation with additional sanctions," the White House announced in a statement about the telephone call between Merkel and Obama.

The conversation between the two leaders also served to discuss next week's four-way meeting of foreign ministers and the EU's foreign policy chief, Catherine Ashton, on the crisis in Ukraine in Geneva.

Both leaders also called again on Russia to withdraw its troops "back from the border region," according to the White House statement.

Earlier on Thursday, NATO Secretary General Anders Fogh Rasmussen also turned up the pressure on Russia, asking it to withdraw its troops. "We have seen the satellite images day after day. Russia is stirring up ethnic tensions in eastern Ukraine and provoking unrest," Rasmussen said, warning that Russian military entry into eastern Ukraine would lead to hard economic sanctions and isolation.

He also accused Russia of using its military might to "dictate" what Ukraine should become, adding that this was "a decision which only Ukraine as a sovereign state can make," he added.

US Treasury Secretary Jacob Lew also weighed in on the debate, telling his Russian counterpart, Anton Siluanov, that "the United States is prepared to impose additional significant sanctions on Russia if it continues to escalate the situation in Ukraine" the US Treasury said.

The comments came as the International Monetary Fund (IMF) and the World Bank kicked off their spring meetings in Washington. The G7 and G20 finance ministers also met, with Ukraine's financing needs on the agenda.

Meanwhile, IMF chief Christine Lagarde said she expects the IMF's board to consider a $14-18 billion two-year bailout fund for Ukraine's stricken economy by early May at the latest. Lagarde also said the Ukraine crisis was one factor that could dampen a global recovery.

World Bank President Jim Yong Kim warned that the crisis could plunge Russia into recession this year."This is a very serious issue for Russia - a very serious issue for its growth prospects," Kim said. "So we simply urge all of the parties to continue with negotiations and find a peaceful means of moving forward."

ng/jm (Reuters, dpa, AP, AFP)