The United Nations Security Council on Sunday urged Yemen's Huthis to cede power, release President Abedrabbo Mansour Hadi and negotiate in "good faith," after the Shiite militia vowed to defy the body's "threats."

Yemen is a traditional U.S. ally in the fight against al-Qaida, but the impoverished Arabian Peninsula country has descended into chaos since the Huthis overran the capital in September.

Fighting dropped sharply across eastern Ukraine on Sunday but there was still sporadic shooting after a ceasefire came into force, the first step in a fragile peace plan aimed at ending 10 months of conflict.

An elderly man and woman died after a Grad missile fire hit the town of Popasna in the Lugansk region some 20 minutes after the truce began at midnight (2200 GMT Saturday), pro-Kiev local governor Gennadiy Moskal said.

Looking east to the bloody conflict gripping Ukraine, NATO-member Poland has kicked off an unprecedented military spending spree worth billions to overhaul its forces as Warsaw believes peace in Europe is no longer a given.

The escalation of tensions with Russia over its annexation of Ukraine's Crimean peninsula and subsequent role in the crisis in that country's east has sounded the alarm on NATO's eastern flank in countries that were under Moscow's thumb during the Cold War.

A ceasefire in Ukraine was cautiously observed by both sides early Sunday, despite accusations by Kiev and the U.S. that Russia had fueled a final push by rebels to gain territory before the deadline.

Ukrainian President Petro Poroshenko ordered troops to abide by the truce from midnight (2200 GMT), in line with a deal reached in Minsk earlier this week with the leaders of Russia, Germany and France.

Both Ukraine and pro-Russian rebels on Saturday pledged to respect a ceasefire due to come into force within hours, after fierce fighting plunged the truce into doubt. 

Under a peace plan signed between Kiev and the rebels in Minsk this week the ceasefire is scheduled to start in war-torn east Ukraine at 2200 GMT on Saturday. 

The separatist guard at a crossing point between Ukraine and Russia poured scorn on Kiev regaining control despite a new peace deal. "I don't think so, we'll stay right here," he said.

The pro-Russian rebels control around 400 kilometres (250 miles) of Ukraine's border with Russia, and Kiev resuming control was one of the toughest negotiating points at Thursday's Minsk summit that resulted in a peace roadmap for eastern Ukraine.

The U.S. said Friday that France's multibillion-dollar sale of Rafale fighter jets to Egypt was no cause for worry, despite Cairo's close defense-sales relationship with America.

"Egypt is a sovereign country," State Department spokeswoman Jen Psaki said. "We have our own security relationship, so I wouldn't say there's a concern from this end."

The United States charged Friday that Russia was continuing to deploy heavy weapons in eastern Ukraine, before a ceasefire agreed this week goes into effect.

State Department spokeswoman Jen Psaki said the United States had received reports of heavy weapons being moved into eastern Ukraine from Russia over the past few days, and more apparently on the way.

Fighting raged in Ukraine on Friday as the clock ticked down to a ceasefire that will be a first test of the commitment by Kiev and pro-Russian separatists to a freshly-inked peace plan.

At least 27 civilians and soldiers died in shelling as German Chancellor Angela Merkel warned Russia it risked fresh sanctions if the fighting did not stop.

Russia wanted an immediate ceasefire in Ukraine and the Sunday morning deadline was agreed in talks in Minsk under pressure from rebel leaders, President Vladimir Putin's spokesman was quoted as saying Friday.

Russia "supported an immediate ceasefire," Putin's spokesman Dmitry Peskov was quoted as saying by the Kommersant business daily.