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.

At least 1,500 Russian troops and convoys of military hardware entered Ukraine over the weekend, the Ukrainian military said on Monday.

"One thousand five hundred Russian soldiers and 300 pieces of military equipment, including Grad missile systems, crossed the Ukraine-Russia border on February 7 and 8," Ukraine military spokesman Andriy Lysensko told journalists.

German Chancellor Angela Merkel was Monday to brief U.S. President Barack Obama on the latest peace plan for Ukraine ahead of a four-way summit aimed at ending 10 months of bloodshed.

Foreign ministry officials from Ukraine, Russia, Germany and France were due to hold a preparatory meeting in Berlin after leaders agreed to push for the key meeting on a new peace deal Wednesday in Minsk, the capital of Belarus.

Ukraine's security service arrested a journalist on treason charges Sunday after he posted a video online urging people to dodge the country's new military draft, his wife and officials said.

Ruslan Kotsaba -- a television journalist from the western region of Ivano-Frankivsk -- was ordered held in custody for 60 days pending investigations, his wife Uliana wrote on Facebook.

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.

Liliya had only just nipped out when the shells tore through her apartment block in Ukraine's rebel bastion Donetsk early Sunday. 

"It's a miracle that I am still alive," the 62-year-old resident of Sportyvna Street told Agence France-Presse.

A system malfunction forced the pilots of an AirAsia X flight from Kuala Lumpur to Jeddah to abort the flight and return to Malaysia on Sunday, the AirAsia Group's CEO said.

The incident sparked fresh questions on social media about the Malaysian airline group's safety record as it struggles with the aftermath of the loss of Flight QZ8501 in late December with 162 people on board.

The leaders of Ukraine, Germany and France are pushing for a summit with Russian President Vladimir Putin on Wednesday in a frantic bid to halt escalating bloodshed in eastern Ukraine.

The four leaders discussed the meeting in a phone call Sunday as part of their efforts to achieve a "comprehensive settlement" in the conflict between Ukrainian forces and pro-Russian rebels, Berlin said.

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.

Seven civilians and five soldiers were killed in fighting in east Ukraine, government and rebel officials said Saturday, with Kiev accusing pro-Russian separatists of massing forces for fresh offensives.

Rebels are "accumulating forces for further offensive operations on Debaltseve and Mariupol," Ukrainian defense official Volodymyr Polyovyi told journalists.