U.S. President Barack Obama called his Russian counterpart Vladimir Putin to discuss escalating violence in eastern Ukraine and "Russia's ongoing support for the separatists there," the White House said Tuesday.

During the call Obama reiterated America's support for Ukraine's "sovereignty and territorial integrity," National Security Council officials said.

Russian President Vladimir Putin met his Egyptian counterpart and agreed a deal to build a nuclear plant Tuesday as he sought to boost ties on his first visit to Cairo in a decade.

The two-day visit came with Moscow bidding to strengthen relations with the most populous Arab country at a time when Cairo's alliance with Washington has frayed.

President Vladimir Putin arrived on Monday in Egypt as Russia seeks to expand its reach in the Arab world's most populous country at a time when Cairo-Washington ties remain frayed.

Putin's first visit to Egypt in a decade comes after a 2011 popular uprising that ousted ex-strongman Hosni Mubarak, whom he leader met on his previous trip in 2005.

Russia said Monday that President Vladimir Putin would not tolerate any ultimatums over Ukraine after a report said Germany had given him until Wednesday to agree a peace plan or face new sanctions.

"We've already said everything about the tone of the negotiations," Putin's spokesman Dmitry Peskov said on Russian radio.

Russian President Vladimir Putin visits Egypt on Monday as Moscow looks to expand its reach in the Arab world's most populous country at a time when Cairo-Washington ties remain frayed.

His two-day trip will be Putin's first in a decade to Egypt and comes after a 2011 popular uprising that ousted ex-strongman Hosni Mubarak, who the Russian leader met in his previous visit in 2005.

Russian President Vladimir Putin is behaving like a "mid-20th century tyrant", British Foreign Secretary Philip Hammond said Sunday, urging him to change track and save Russia's economy from further decline.

He told Sky News television that supplying arms from Britain to Ukraine would not be the right thing to do "at the moment", but insisted London's position would be kept under review.

The United States is weighing "additional" assistance to Ukraine as it battles Russian-backed separatists, Secretary of State John Kerry said Saturday, while stopping short of saying Washington would send arms.

Kerry's comments came as Western leaders challenged Russian President Vladimir Putin to prove he wants peace in Ukraine, warning both sides a new Franco-German peace drive may be a "last chance" to stop all-out war.

German Chancellor Angela Merkel on Saturday pushed a new peace bid to end the Ukraine conflict, warning that sending weapons to government forces fighting pro-Russian rebels would not stop the bloodshed.

Fresh from overnight talks in Moscow with French President Francois Hollande to thrash out a new peace plan, Merkel said success was far from assured but it was essential to try.

German Chancellor Angela Merkel and French President Francois Hollande met Russian leader Vladimir Putin Friday in a frantic bid to avoid a further escalation of violence in east Ukraine.

Ahead of the Moscow talks, Merkel played down hopes of a rapid end to surging fighting as she and Hollande try to convince Putin to sign up to a peace plan to stop the conflict.

President Vladimir Putin's spokesman has angrily dismissed a Pentagon study that claimed the Russian leader had Asperger's syndrome, a form of autism.

"That is stupidity not worthy of comment," spokesman Dmitry Peskov told Gazeta.ru news website late Thursday.