U.S. and Russian officials were to meet Saturday on Syrian opposition charges of truce violations by the regime but peace talks remain on course, U.S. Secretary of State John Kerry said.

Asked at the end of a visit to Saudi Arabia whether indirect dialogue in Geneva between the Syrian regime and opposition could go ahead as planned on Monday, Kerry said: "Yes, it can."

The chief prosecutor for the Yugoslav war crimes tribunal said Friday that those behind atrocities in Syria must eventually be held to account, as the conflict prepares to enter its sixth year.

"As an international prosecutor and somebody who believes in justice... it is obvious that sooner or later accountability will be needed for the crimes committed in Syria," Serge Brammertz told AFP.

Russia's Foreign Minister Sergei Lavrov on Friday urged the U.N.'s special envoy on Syria to include Kurds in upcoming talks that aim to resolve the five-year Syrian civil war.

"I am convinced that Staffan de Mistura should take such a decision," Lavrov told reporters at a joint news conference with his Chinese counterpart.

Syria's fifth year of war was the worst yet for civilians increasingly beset by aid blockades and sieges, humanitarian groups said Friday ahead of the anniversary of the uprising.

"Russia, the United States, France and the United Kingdom must now safeguard the glimmer of hope that the ceasefire has brought to civilians, rather than 'adding fuel to the fire'," the report said, referring to a partial truce that has largely held since February 27.

Syria's main domestic opposition body Thursday called for a widespread boycott of parliamentary elections next month, accusing the government of using the vote to gain leverage in peace talks.

The Islamic State group's battle-tested equivalent of a defense minister is believed to have been killed in a U.S. air strike in northeastern Syria, a U.S. official here said.

The target of the March 4 attack was Omar al-Shishani, a red-bearded Georgian fighting with the jihadist group in Syria, the Pentagon said Tuesday, cautioning that results of the operation were still being assessed.

Russia's defense ministry on Monday said it would open up its military facilities in Syria to international aid deliveries in a bid to bolster humanitarian assistance in the war-torn country.

"The ministry is ready to provide necessary assistance to international and foreign organizations in delivering humanitarian cargoes to the Syrian Arab Republic," it said in a statement.

U.S. Vice President Joe Biden said Monday Washington was going to have to "squeeze the heart out of" the Islamic State group in Syria and Iraq to wipe it out.

"We have to squeeze the heart of Daesh in Iraq and Syria so they can't continue to pump the poison in the region and the rest of the world," he said, using an Arab acronym for IS.

Regions of Syria covered by a U.N.-brokered ceasefire Sunday enjoyed the "calmest day" since the truce started, a monitor said, as Moscow and Washington cautioned against any delay in resuming peace talks.

"Sunday was the calmest day since the ceasefire came into effect" on February 27, Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, told AFP.

French President Francois Hollande on Friday criticized his Syrian counterpart Bashar Assad for scheduling elections in his war-torn country next month, calling the move "provocative" and "unrealistic".

"The idea that there could be elections (in April) is not just provocative but totally unrealistic," Hollande said after meeting in Paris with German Chancellor Angela Merkel.