BEIRUT (AP) — Al-Qaida militants massed on Monday near a Syrian border town, edging closer to a vital crossing under control of Western-backed rebels, activists said, underscoring the weakness of the fighters who America hopes could be a moderate force in the chaotic civil war.

It was not clear whether the al-Qaida-linked Nusra Front would attempt to take the Bab al-Hawa crossing after the militants overran towns and villages this week held by Western-backed groups in the northwestern Idlib province. Reports later emerged that some Western-backed rebels pledged allegiance to the al-Qaida group.

Idlib province was a “core stronghold” for the Western-backed groups “and they have been most definitively defeated in a few days,” said Charles Lister, a visiting fellow at the Brookings Doha Center.

“It’s very significant.” Lister said. “It shows that … the moderates are weaker than what we have been led to believe, despite increased funding from the West.”

There was no evidence that the Islamic State group, which has seized a third of both Iraq and Syria, and the Nusra Front, Syria’s main al-Qaida branch, acted in unison in the latest push. The two are bitter rivals who repeatedly have battled each other.

Rather, it appeared that the Nusra Front sought to control a key supply line to the Syrian rebels fighting against President Bashar Assad’s rule.

Meanwhile, the Islamic State group has been focused on trying to seize the eastern Syrian border town of Kobani from Kurdish fighters for more than a month. A U.S.-led coalition targeting the extremists launched four airstrikes Sunday and Monday, targeting its fighters near Kobani and one near Dayr Az Zawr, U.S. Central Command said.

Amid the fight for Kobani, Nusra Front fighters have been gathering for the past few days in the town of Sarmada in the northern Idlib province, some 6 kilometers (4 miles) from Bab al-Hawa, said Assad Kanjo, an anti-Assad activist based in the province.

Rami Abdurrahman of the Britain-based Syrian Observatory for Human Rights also said Nusra fighters had been seen gathering in Sarmada, but said there was no indication they had advanced on the crossing.

The Bab al-Hawa crossing is held by a rebel alliance known as the Islamic Front, and it is an important supply route for Western-backed fighters as well as aid groups to reach residents of northern Syria.

Lister said he did not believe Nusra fighters would attack the crossing, saying they were unlikely to directly challenge the Islamic Front, a collection of hard-line and moderate Muslim groups. He said that likely would create new, unnecessary enemies for the group.

“Most likely, they will seek to consolidate their influence in the area around Bab al-Hawa,” he said.

The recent defeats were a sharp blow for the Western-backed Syrian Revolutionaries Front, headed by prominent commander Jamal Maarouf, and Harakat Hazm, a group armed and funded by the U.S.

Harakat Hazm emerged earlier this year and online videos have showed its fighters using Western-donated weapons, including U.S. anti-tank weapons in the spring. It has made modest advances in Idlib.

But the U.S. aid never increased and it couldn’t compete with stronger militant groups, Lister said.

Maarouf has emerged as a wily commander who has obtained support of Western backers. He presents himself as a moderate, but has used sharp sectarian language to refer to the Syrian army.

In an online video posted Saturday after his forces were pushed out of parts of Idlib, Maarouf accused Nusra fighters of “occupying” Idlib and compared them to Assad’s forces.

In the messy alliances that have characterized the Syrian war, the Nusra Front has fought alongside other rebel groups against Assad’s forces. But it has particularly poor relations with Maarouf’s men. They previously fought over the summer over what appeared to be to rights to smuggling Syrian fuel to Turkey.

The Syrian civil war, which began as a peaceful uprising against Assad’s rule more than three years ago, has become a bloody and protracted sectarian conflict that has killed more than 200,000 people, activists say.

That anger over rebel in-fighting could be seen in an online video showing a man standing in a recently bombed olive grove in Idlib. Some areas of Idlib are still being shelled by Assad’s forces and the video, as well as the others, appeared genuine and corresponded with Associated Press reporting.

Clutching shrapnel from the olive grove, the man said: “You are fighting over positions and the people are (buried) in the ground.”