The attack appeared to signal another new territorial gain for the breakaway
The conquests have highlighted the militant group's continued potency despite fighting pro-government forces in both Syria and Iraq and enduring months of U.S.-led airstrikes.
Pro-opposition activists hostile to Islamic State said Sunday that the militants in their latest thrust had overrun the village of Soran, east of the Syrian border town of Azaz, and nearby areas.
Islamic State forces were widely expected to assault Azaz, which has been in the hands of insurgents hostile to Islamic State for more than a year.
The town is a strategic prize because of its proximity to the giant Bab al-Salam border crossing into Turkey, a major transit point for goods and supplies entering northern Syria. Turkey, which shares a more than 500-mile border with Syria, is a vital logistics and resupply hub for a broad range of armed Syrian opposition groups.
Since Syria's conflict began in 2011, various rebel factions have vied for control of border posts. Dominating a frontier zone can be a strategic and financial windfall. Factions in control of border posts are positioned both to supply their forces expeditiously and to collect fees from truckers and others entering Syria.
Fierce resistance from Kurdish-led forces aided by U.S. bombing attacks early this year helped thwart Islamic State's bid to seize the Syrian border town of Kobani, about 90 miles northeast of Azaz. Kurdish and allied forces also are reported to be fighting to evict Islamic State from the crucial Tal Abyad border crossing, about 40 miles southeast of Kobani.
On Sunday, the Islamic State advance toward Azaz triggered widespread alarm among rival rebel factions in social media postings.
The Aleppo Revolutionaries Union, a pro-opposition body hostile to Islamic State, issued a statement calling on all its allies to confront the militant group after "its vicious attack."
The rebels arrayed against Islamic State in northwestern Syria include both hard-core Islamist and some Western-backed factions. Prominent among them are hard-line Al Qaeda-linked groups such as Al Nusra Front and Ahrar al Sham -- both ultra-conservative Sunni Muslim groups that are fierce rivals of Islamic State.
Many predicted that the Islamic State thrust would probably push back opposition plans for a renewed offensive against government-held areas in the city of Aleppo, which has long been divided between opposition and loyalist forces. Various insurgent factions opposed to Islamic State had been heralding an impending assault on Aleppo.
"All the preparation work for the liberation of Aleppo has now been sabotaged by the attack," an Ahrar al Sham spokesman who goes by the nickname Abu Yusuf al-Muhajer said via Skype from Turkey.
In its latest advance, Islamic State appears to be seeking to recapture territory that it lost early last year, when it was expelled from Azaz and much of Aleppo province in fighting. But Islamic State has maintained a foothold in the province.
Syrian state-run media said Sunday that rebel shelling had killed 12 civilians in government-held areas of the city of Aleppo, the latest such attacks. The opposition has said that scores of civilians have perished in government bombing of rebel-controlled areas of Aleppo.
Elsewhere in Syria, state media and other reports indicated that at least 25 people, including children, were killed when a fuel tank exploded at a clinic in the predominantly Kurdish northeastern city of Qamishli, which has largely been spared the violence seen elsewhere in Syria. Reports indicated that the blast occurred when children were at the site receiving polio vaccinations. The cause of the fuel tank explosion was not immediately clear.
The more than four-year Syrian conflict has claimed more than 200,000 lives, according to the United Nations, and left much of the country destroyed or in the hands of armed Islamist factions. All efforts for a diplomatic solution to end the war have so far failed.
Bulos is a special correspondent.