Militants from the so-called "Islamic State" (IS) claimed responsibility for the series of blasts which went off at almost the same time across Syria on Monday. At least 40 people died in cities controlled by the government and in areas under the control of Syrian Kurdish militia.

The biggest blast was in the coastal city of Tartus (photo above) where support for the government is strong. A highway checkpoint was targeted in a first blast which was followed by a second explosion, apparently directed at rescue workers. The city is a popular beach resort for Syrians and the site of a leased military installation of the Russian Navy.

The other blasts went off in a highly guarded Damascus suburb housing military officers and their families, a government-held area in the city of Homs and in the cities of Qamishli and Hasakeh, in the Kurdish-controlled northeast.

IS border setback

Syrian rebels, backed by Turkish warplanes and tanks, had pushed IS fighters out of the border strip on Monday, securing their hold on an area seized over the weekend. The operation deprived IS of a key supply and transport corridor. It also prevented Kurdish militia from establishing a semi-autonomous area.

The IS attacks could be the start of a new phase in their battle with the Syrian government, according to Jennifer Cafarella, a Syria expert with the Washington-based Institute for the Study of War.

"It's a dangerous possibility that we're witnessing ISIS gear up for a campaign to expand westward into either or both regime and opposition territory as it loses to the anti-ISIS coalition," she said in an interview with the Associated Press, using another acronym for the militant group.

US backs Kurdish militia

Syrian Kurdish militia in the region have received support in their operations against IS fighters. The US envoy to the region, Brett McGurk, visited Kurdish-led, US-backed forces in northern Syria on Sunday, according to the State Department.

McGurk confirmed US support but also warned the Kurds they had to hold to a commitment to remain east of the Euphrates River.

West of the key river, Syrian rebels backed by Turkey have established control over an area which Ankara appears to want to use as a buffer zone and refuge for displaced Syrians.

On Monday, Turkish President Recep Tayyip Erdogan said he had proposed to the US and Russian leaders that a no-fly zone be established there.

Russian airstrikes

Also on Monday, Russian airstrikes supported efforts by the Syrian government to take territory on the outskirts of the northern city of Aleppo.

Russian and US officials were still trying to come to terms for allowing humanitarian aid into the besieged parts of the city.

In China for the G20 meeting, Presidents Barack Obama and Vladimir Putin held discussions about the Syrian conflict but failed to come to any agreement.

Obama described his talks with Putin as "productive," but said they were challenged by "gaps of trust."

jm/cmk (AP, AFP, Reuters)