Damascus: Explosions in government-controlled areas of Syria and a province held by Kurdish militia killed dozens on Monday, while the United States and Russia failed to make concrete progress towards a ceasefire.

Six explosions hit west of the capital Damascus, in the government-held cities of Homs and Tartous - which hosts a Russian military base - and the Kurdish-controlled northeastern province of Hasaka between 8 a.m. and 9 a.m. (0500-0600 GMT), state media and a monitor said.

It was not clear if the blasts were linked and there was no immediate claim of responsibility.

More than five years of civil war have cut Syria into a patchwork of territories held by the government and an often competing array of armed factions, including Kurdish militia fighters, a loose coalition of rebels groups, and Islamic State.

The United States and Russia have been trying to broker a new truce after a cessation of hostilities agreed in February unraveled within weeks, with Washington accusing Syrian President Bashar al-Assad's forces of violating the pact.

Their efforts were complicated on Sunday as government forces and their allies again laid siege to rebel-held eastern Aleppo, Syria's largest city before the war which Assad is determined to fully recapture. His gains have relied heavily on Russian air support since September last year.

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U.S. President Barack Obama and Russian President Vladimir Putin had a longer-than-expected discussion of about 90 minutes on Monday about whether, and how, they could agree a deal, a senior U.S. administration official said.

Meeting at the G20 summit in China, they discussed getting humanitarian aid into the country, reducing violence, and cooperating on combating militant groups, the official said.

But in talks earlier on Monday, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov were unable to come to terms on a ceasefire for the second time in two weeks, with U.S. officials stressing they would walk away if a pact could not be reached soon.

"If an agreement can be reached, we want to do so urgently, because of the humanitarian situation. However, we must ensure that it is an effective agreement," the official said.

Russia says it cannot agree to a deal unless opposition fighters, backed by the United States and Middle East allies, are separated from al Qaeda-linked militants they overlap with in some areas.

For Washington, the priority is stabilizing Syria so as to destroy Islamic State, which controls territory both there and in neighboring Iraq.

NATO Turkey ally on Sunday said rebels it was backing had gained control of all areas on its border that had been held by the jihadists, depriving the ultra-hardline Islamist group of its main route to the outside world.

The announcement came some 10 days after Turkey launched its first major military incursion into Syria since the start of the war in 2011, an operation aimed as much at preventing further Kurdish territorial gains as at driving back Islamic State.

DOZENS KILLED

Two of the explosions on Monday hit the Arzouna bridge area at the entrance to the Mediterranean city of Tartous, the Syrian Observatory for Human Rights and state news agency SANA said. Islamic State attacked Tartous in May.

The Observatory and a city hospital put the death toll at 35, including members of the Syrian military, and said the number was likely to rise.

Syrian state television said the first explosion was a car bomb and the second a suicide belt detonated as rescue workers arrived. The blasts hit during a summer festival at Tartous, whose beaches recently featured in a government tourism video.

A car bomb meanwhile struck Homs, a city around 80 km (50 miles) east of Tartous which has repeatedly been hit by bombings claimed by Islamic State. State media said three people were killed, while the Observatory said the explosion hit an army checkpoint and four officers were killed.

West of Damascus, there was an explosion near the town of al Saboura, killing one person and injuring three, according to a police commander quoted by state media.

A motorbike also exploded in the centre of the northeastern city of Hasaka, which is controlled by the Kurdish YPG militia.

The Observatory said the blast killed three members of a YPG-affiliated security force known as the Asayish, and injured others. It said a percussion bomb also went off in the province's Qamishli city.

The Kurdish YPG militia, a critical part of the U.S.-backed campaign against Islamic State, took almost complete control of Hasaka city in late August after a week of fighting with the government.

Islamic State confirmed that a blast in Hasaka took place but did not say whether its fighters were involved.

The YPG already controls swathes of northern Syria where Kurdish groups have established de facto autonomy since the start of the Syrian war in 2011, much to the alarm of neighboring Turkey.

Ankara sees the YPG as an extension of Kurdish militants who have waged a three-decade insurgency in Turkey and fears the creation of a Kurdish enclave in northern Syria would fuel their separatist ambitions.