Ukrainian President Petro Poroshenko accused Russia on Friday of challenging the "entire democratic world" by annexing Crimea and allegedly arming separatist rebels in the east of his ex-Soviet state.

The pro-Western leader also called on Ukraine's Western allies to toughen their trade and other sanctions on the Kremlin and its biggest firms, should Moscow fail to fulfill the terms of a tentative peace deal to the 17-month war.

Russia has lifted its objections to a U.N. investigation into chemical attacks in Syria, clearing the way for the probe to begin, diplomats said Thursday.

The U.N. Security Council unanimously adopted a resolution August 7 approving a joint investigation by the United Nations and the Organization for the Prohibition of Chemical Weapons.

Assassinated Russian opposition leader Boris Nemtsov, a Ukraine military pilot held on murder charges by Moscow and jailed Saudi Arabian blogger Raif Badawi were shortlisted Thursday for the European Parliament's prestigious Sakharov human rights prize.

The prize is awarded every year to honor individuals who combat intolerance, fanaticism and oppression, often falling foul of their governments as a result.

Russians will vote Sunday in a regional election expected to yield few surprises, with the country's liberal opposition only able to field a handful of candidates.

Voters in 42 regions will elect governors and lawmakers for regional and city assemblies, and in some cases will cast ballots for both. 

A defiant Russia on Thursday said it was ready to increase military support to Syria's Bashar Assad if needed, dismissing suggestions it could hamper Western efforts to fight Islamic State militants.

Washington has accused Moscow of solidifying its foothold in Syria, raising concerns it could throw a wrench in plans by the U.S.-led coalition to step up operations against jihadists in the war-torn country. 

Photos allegedly showing Russian soldiers, reports of military deliveries and overflight requests are swelling fears among the United States and its allies that Russia is covertly bolstering the Syrian regime.

Moscow, an ally of Syria's President Bashar al-Assad, has had a military presence in the country since the Soviet era.

Ukraine's army said on Wednesday that a new nine-day truce with pro-Russian insurgents was holdings despite small skirmishes that claimed the lives of two government soldiers this week.

"A relative calm on the front is holding," Ukrainian military spokesman Andriy Lysenko told reporters.

Syria has denied reports of increased military activity by Russian troops on its soil, after Washington said it was following up on claims of ramped up support from Moscow.

Speaking late Monday to Hizbullah's al-Manar television station, Information Minister Omran Zohbi dismissed the reports as baseless.

Eight meters long and weighing 25 tonnes, a replica of the so-called Tsar Bomb, the most powerful nuclear bomb ever detonated, has gone on display for the first time in Russia, in the midst of an ongoing standoff with the West over Ukraine.

Tested in 1961 by the Soviet Union, the hydrogen bomb -- also known as the AN602 -- instilled a mix of pride and fear in retired military pilot Nikolay Krylov as he looked at the replica housed at an exhibition center near the Kremlin. 

U.S. Secretary of State John Kerry raised U.S. concerns about reports of "an imminent enhanced Russian military buildup" in Syria, in a phone call Saturday to his counterpart in Moscow, the U.S. State Department said.

"The secretary made clear that if such reports were accurate, these actions could further escalate the conflict, lead to greater loss of innocent life, increase refugee flows and risk confrontation with the anti-ISIL coalition operating in Syria," the State Department said.