The attack in Tartus struck a checkpoint along a highway that runs from the coastal areas, where support for the government is strong, to the Syrian capital, Damascus, connecting a string of major cities that the government has made it a priority to control. An initial blast was followed a few minutes later by another, apparently an attempt to target rescue workers.

Around the same time on Monday morning, blasts went off in a government-held area in the central city of Homs and in two cities, Qamishli and Hasakeh, in the Kurdish-controlled northeast. The bombs in the Kurdish areas were quickly claimed by the Islamic State’s media agency, Amaq.

Those bombings occurred after major setbacks for the Islamic State, which on Sunday was pushed from its last footholds along the Turkish border by Turkish forces and Syrian rebels backed by Turkey and the United States. The changes have denied the Islamic State a key supply and transportation corridor while allowing Turkey to block the establishment of a Kurdish semiautonomous zone there. They have also given a lift to Syrian rebels eager to show they can help international efforts against the Islamic State.

But the changes sent American officials scrambling to mend fences with the Syrian Kurdish militias, which also receive American assistance against the Islamic State. The State Department said Brett H. McGurk, the United States envoy to the region, had visited Kurdish-led American-backed forces in northern Syria recently and given assurances that American support was still strong. But he also issued a warning that the Kurds must abide by commitments to remain east of the Euphrates River.

West of the river, Syrian rebels and their Turkish backers now control an area that Turkey has long coveted as a buffer zone, where displaced Syrians, protected from aerial bombing, could gather instead of flowing into Turkey. Turkey’s president, Recep Tayyip Erdogan, declared Monday at the Group of 20 summit meeting in China that he had proposed to President Vladimir V. Putin of Russia and President Obama that a no-fly zone be established there.