Forces of Syria's President Bashar al-Assad remove the flag of al-Qaeda-linked Jabhat al-Nusra, which they said was left behind by rebel fighters, in Zor al-Mahruqa village on Oct. 6. (George Ourfalian/Reuters)

U.S. officials are weighing whether to broaden the air campaign in Syria to strike a militant group that is a rival to the Islamic State and that is poised to take over a strategically vital corridor from Turkey.

Extremists from the al-Qaeda-linked Jabhat al-Nusra group were said Monday to be within a few miles of the Bab ­al-Hawa crossing in northwestern Syria on the Turkish border, one of only two openings through which the moderate Free Syrian Army receives military and humanitarian supplies provided by the United States and other backers.

Over the weekend, rebels said Jabhat al-Nusra forces swept through towns and villages controlled by the Free Syrian Army in Idlib province, west of Aleppo. Rebel groups associated with the Free Syrian Army were routed from their main strongholds, with scores of fighters fleeing toward Turkey or defecting to join the militants, according to opposition activists.

Apart from one attack by Tomahawk missiles against an ­al-Qaeda cell within Jabhat al-Nusra in late September,when the Syrian airstrikes began, U.S. and Arab warplanes have been targeting the Islamic State, a separate group that the administration has made clear is its primary target in Iraq and Syria.

The recent fighting in northwestern Syria has been taking place a long way from areas farther east where U.S. and Arab warplanes have been pounding Islamic State positions.

An air strike hit the Syrian town of Kobane on Sunday, as Iraqi Kurdish fighters battled Islamic State militants. A large plume of smoke could be seen rising from the Syrian town shortly after the strike was heard. (Reuters)

But U.S. concern has grown rapidly in recent days amid fears about the border crossing, according to senior administration officials who spoke about internal discussions on the condition of anonymity.

Officials cautioned that no proposal for expanded airstrikes has reached the level of decision-making, and its main advocates may not include the White House.

“There are a lot of possibilities that are always being discussed. I don’t want to make it seem like we’re on the verge of an announcement,” one senior administration official said. “We want to help the opposition, we want to keep the border crossings open, and we’re looking a lot of things,” the official added. But “all of those actions are immensely complicated, for reasons you can imagine.”

Among the complications are Syrian government air defenses. President Bashar al-Assad, whom both the rebels and the militants are trying to overthrow — even as they fight each other — has not interfered in the U.S.-led air campaign against the Islamic State in areas of northern and eastern Syria. Nor has the government aggressively contested them there. It is unclear whether Assad would tolerate an expansion into other areas.

The U.S. focus on the Islamic State has angered the Free Syrian Army, which believes that the airstrikes are indirectly assisting Assad. And despite the battles last weekend, some elements of the moderate opposition still consider Jabhat al-Nusra an ally in their fight against Assad.

The administration has repeatedly insisted that its long-term goal is the removal of Assad. But it considers the threat from the Islamic State, which has spread across a third of Iraq in addition to northern and eastern Syria, to be an emergency and the first order of business.

Although President Obama’s strategy includes stepped-up military training and equipment for the Syrian rebel fighters, that program is expected to take the better part of a year and won’t get underway for months.

Free Syrian Army frustration with what it sees as slow and insufficient assistance was exacerbated late last month, when retired Gen. John Allen, the administration’s coordinator for the international coalition battling the Islamic State, said that the U.S. training and weapons were intended to help the rebel group fight the militants, not Assad.

“There could be FSA elements that ultimately clash with the regime” in a defensive capacity, Allen told Asharq al-Awsat, a leading Arab newspaper. “. . . But the intent is not to create a field force to liberate Damascus — that is not the intent.”

Although Allen later clarified and softened his remarks, bad feelings remain among the rebels and their Sunni Arab backers. Countries including Saudi Arabia and the United Arab Emirates, which are assisting in airstrikes against the Islamic State, suspect that the administration is going easy on Assad to avoid antagonizing Iran, a key Assad supporter, while it negotiates a separate nuclear deal with the Tehran government and welcomes Iranian assistance against the Islamic State in Iraq.

Despite rising U.S. concern about the border crossing, disagreements between the rebels and the administration extend to what happened over the weekend.

“We are aware of press reports of a major al-Nusra offensive against the Free Syrian Army, but I have nothing confirming this,” said U.S. Central Command spokesman Maj. Curtis J. Kellogg.

A key official with the Syrian Opposition Coalition, the U.S.-backed political umbrella for the Free Syrian Army, said Monday that it has appealed for three days to the administration for emergency assistance.

“We sent a note to Gen. Allen’s office on Friday” requesting coordinated airstrikes and expedited military aid, the opposition official said. “We were told that the information was passed on to Centcom. . . . No response.”

Oubai Shahbandar, a spokesman for the opposition coalition, said the moderate rebels still hold some ground near the border. “All is not lost,” he said, speaking by telephone from Istanbul. “The Nusra guys have made some pretty significant advances. . . . What happens in the next few days will be really crucial.”

The senior Obama administration official said he was “not aware” of any emergency request for assistance. But, the official said, “you can imagine in situations like this that the Syrians and Iraqis make requests all the time.”

Liz Sly in Beirut contributed to this report.