BEIRUT—Al Qaeda militants massed on Monday near a border town in Syria, 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 Qaeda-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 Qaeda 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 Qaeda branch, acted in unison in the latest push. The two are bitter rivals who repeatedly have battled each other.

Rather, it appeared 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 strike 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 six kilometres 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.

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