BEIRUT — Al Qaeda-linked militants pressed an offensive Monday against Western-backed rebels in northern Syria, closing in on a vital border crossing with Turkey and exposing the weakness of mainstream opposition groups that the United States hopes to forge into a fighting force to take on Islamic extremists.
The Nusra Front’s recent surge has overrun strongholds in Syria’s Idlib province of two prominent rebel factions that proved unable to repel the assault despite getting arms and training from the United States.
The opposition groups’ collapse marks a significant setback to Washington’s plan of partnering with more moderate brigades to fight the Islamic State group and other radicals.
The Nusra Front — a bitter and bloody rival of the Islamic State group despite their shared extremist ideology — was massing its fighters Monday in the town of Sarmada near the Bab al-Hawa border crossing after sweeping through rebel-held towns and villages during the weekend.
As the extent of the rout became apparent, reports also emerged that some rebels had pledged allegiance to the Al Qaeda affiliate.
The fighting takes place against the backdrop of US-led airstrikes to roll back and destroy the Islamic State group, which has seized huge chunks of territory spanning Syria and Iraq. That effort has recently focused on the Syrian Kurdish town of Kobani on the Turkish border.
The two primary targets of the Nusra Front’s attack are the Syria Revolutionaries Front and Harakat Hazm. While both rebel groups have received US support, it never reached the levels that either deemed necessary to make significant advances against President Bashar Assad’s forces in Syria’s 3½-year-old civil war. At the same time, the link to the Americans also earned them the enmity of radical groups.
‘‘The Nusra Front is deeply suspicious of both the SRF and Harakat Hazm because they receive support from the US,’’ said Aron Lund, editor of Syria in Crisis, a website run by the Carnegie Endowment.
‘‘The US is also quite open about training rebels to take on both the Islamic State and Al Qaeda, to which the Nusra Front belongs. So from the Nusra Front’s perspective, these groups aren’t just troublesome rivals, they’re a pro-Western fifth column that are slowly being readied for a purge of jihadis,’’ Lund said.
Tensions between the groups worsened after the United States bombed Nusra Front bases in Idlib province on the opening night of its air campaign focused primarily on the Islamic State group. Those airstrikes won the Nusra Front, which has largely focused on fighting Assad’s forces, the sympathy of many in opposition-held areas.
Mainstream rebel groups maintain a presence in parts of northern Syria and are also active in the south along the Jordanian border. But the quick defeat of two groups previously considered among the stronger factions in Idlib province ‘‘shows that . . . the moderates are weaker than what we have been led to believe, despite increased funding from the West,’’ said Charles Lister, a visiting fellow at the Brookings Doha Center.
It was unclear how far the Nusra Front intended to take its offensive in Idlib. In recent days, the group’s fighters have been gathering in the town of Sarmada in northern Idlib province, about 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 flowed into Sarmada but added there was no indication they had advanced on the crossing, which is held by a rebel alliance known as the Islamic Front.