Ukraine on Thursday renewed accusations of a Russian military build up on its border and approved fresh troop mobilizations as a wave of violence threatened all-out conflict in the country's war-torn east.

A national day of mourning was held for 13 people killed on Tuesday when a rocket exploded near a commuter bus traveling towards the Ukrainian city of Donetsk, the worst loss of civilian life since a September truce that only partially halted the violence.

The EU should consider leaving the annexation of Crimea out of the debate over sanctions as part of a move to re-engage with Russia, according to a policy paper seen by AFP Thursday.

The paper drawn up by the office of the bloc's foreign policy chief Federica Mogherini ahead of talks between the 28-nation EU's foreign ministers next week proposes a new way forward to try end the nearly year-long standoff between Russia and the West over Ukraine.

It was a simple gesture of politeness that saved her: A young man offered her a seat on a bus transporting Ukrainians in search of pension payments in the war-torn east, but she remained standing.

"And today he is dead," Olga Kukunyuk, 31, said at a hospital in Volnovakha, where she was being treated for wounds to her arm and leg, before breaking down in tears. "If I had been seated in his place, I would be dead."

Russia launched a probe into attempted murder after a mysterious blast on Thursday injured two people at a bus stop in the Siberian city of Angarsk, local officials said.

A man was hospitalized and a woman was lightly wounded, regional law enforcement officials said in a statement, adding they received the first reports of a "loud bang" at 8:00 am local time (midnight GMT).

Hundreds of angry Armenians protested near a Russian military base Wednesday, demanding that a soldier accused of murdering a family of six be delivered to Armenian justice authorities for trial.

Russian conscript Valery Permyakov is suspected of having shot dead an entire family in the city of Gyumri, including a two-year-old child. Another infant was gravely injured.

Russian Foreign Minister Sergei Lavrov warned Kiev on Wednesday against using force in the conflict in eastern Ukraine, where violence rose sharply this week.

"We are warning our Ukrainian colleagues against attempts to once again return to the scenario of force," Lavrov said at a press conference in Moscow.

U.S. Secretary of State John Kerry Wednesday voiced backing for a Russian-led bid for new talks to end Syria's devastating conflict.

Kerry, who is in Geneva for discussions with his Iranian counterpart to accelerate a deal on reining in Tehran's nuclear programme, met with U.N. peace envoy Staffan de Mistura for talks on the Syrian crisis and lauded his efforts.

The death toll from a stray rocket that hit a Ukrainian bus climbed to 12 on Wednesday as Moscow and Kiev traded blame for the separatist war's bloodiest incident since a September truce.

The long-range Grad rocket exploded on Tuesday near a commuter bus traveling towards the pro-Russian rebel stronghold of Donetsk from Ukraine's southeast coast.

Inflation in Russia could hit 17 percent this year, the deputy economy minister said Wednesday, as the ruble suffers on the back of slumping oil prices.

"Peak inflation will come in March-April when year-on-year inflation could reach 15 to 17 percent," Alexei Vedev was quoted as saying by the state-run TASS news agency.

The U.N. Security Council strongly condemned on Tuesday a rocket attack on a bus in east Ukraine that left 11 civilians dead.

In a unanimous statement of the 15 members including Russia, the council called for an investigation to bring those responsible for the attack to justice.