Top diplomats from the United States and Russia on Monday failed to reach a deal to ease fighting in Syria as a string of bomb attacks killed at least 43 people there, underscoring the need to stem the violence.

Washington and Moscow support different sides in a festering conflict that has left 290,000 dead, and sent hundreds of thousands of refugees fleeing to Europe.

The US and Russia also co-chair a UN-backed humanitarian taskforce for Syria, which has been struggling to get desperately needed aid into the country.

As blasts maimed and killed in Syria, a senior State Department official said fresh crisis talks between Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov on the margins of the G20 summit in China had ended without agreement.

A deal to provide aid to ravaged civilians in the besieged city of Aleppo, and at least partially halt Russian and Syrian bombardments had looked likely on Sunday.

But US officials accused Russia of backtracking on already agreed issues that Washington refused to revisit.

The talks also appeared to have been overtaken by developments on the ground, with Syrian government troops tightening their grip on Aleppo.

State media said they had taken an area south of the city, severing the last opposition-held route into its eastern neighbourhoods.

Monday's string of explosions hit government-held Tartus and Homs, as well as Hasakeh, which is mostly controlled by Kurdish forces but where the regime of President Bashar al-Assad maintains a presence, according to Syrian state media.

At least 43 people perished and dozens were wounded.

Tartus, which has become a refuge for many Syrians fleeing violence, has been largely spared the worst of Syria's conflict since it began with anti-government protests in March 2011.

The bombings came after advances by Turkish forces and allied Syrian rebels expelled IS from the last stretch of the Syrian-Turkish border under their control.

The jihadist group has been losing ground to both an alliance of Kurdish and Arab fighters known as the Syrian Democratic Forces and, more recently, a Turkish offensive involving rebels loyal to Ankara.

Presidents Barack Obama and Vladimir Putin had a 90-minute meeting Monday on the sidelines of the G20 summit in the Chinese city of Hangzhou, where they agreed to continue talks, a White House spokesman said.

The leaders "discussed efforts to reach an agreement on Syria to reduce violence, provide humanitarian assistance and to focus on Al Qaeda -- also known as Al Nusra -- and (IS) in Syria," he said.

"If an agreement can be reached we want to do so urgently because of the humanitarian situation, however we must ensure it is an effective agreement," he added.

But the leaders did not "get down into the weeds", the official said, saying the remaining disagreements were "technical" and would be discussed in the coming days by Kerry and Lavrov.

Earlier truces in Syria have rapidly deteriorated, and Obama warned Sunday that the US was approaching the talks "with some scepticism".

Washington is also highly reluctant to offer Putin a high-profile stage to gain international legitimacy after his backing for a regime that has used chemical weapons on civilians.

US officials had hoped to build pressure on Moscow over its support for Assad's government during the Hangzhou summit and upcoming UN General Assembly.

The US has repeatedly said that Assad must step down if Syria is to reach a lasting peace.