Al Qaeda-tied fighters reportedly overran Western-backed Syrian rebels in key strongholds over the weekend and threatened to advance deeper into their territory on Monday, dealing a blow to the Obama administration’s efforts to train and equip moderate opposition forces.

White House Press Secretary Josh Earnest insisted Monday that the mission to prop up a competent Syrian fighting force will take time, and is “not a short-term proposition.”

“This is going to require a sustained effort and a sustained commitment,” Earnest said.

But the reported advances by the Al Qaeda-linked Nusra Front underscored the weakness of the fighters America hopes can act as a de facto ground force in Syria against the Islamic State.

The Washington Post reported that U.S.-backed Free Syrian Army rebels surrendered or defected as Nusra fighters trampled towns and communities in the province of Idlib.

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    The Associated Press reports that the Nusra fighters have since amassed near the border town of Bab al-Hawa.

    The Nusra Front and the Islamic State are bitter rivals, and there was no evidence that the two acted in unison in the latest push. It appeared the Nusra Front sought to control a key supply line to the Syrian rebels fighting against President Bashar Assad's rule.

    The Syrian rebels, though, are a key plank of the West’s strategy, together with airstrikes and cooperation from regional partners, to defeat the Islamic State in Syria.

    A former military intelligence officer who keeps ties to Free Syrian Army commanders told Fox News that Nusra fighters have indeed made these advances against Syrian rebels in Idlib. The source told Fox News he has spoken with his contacts in Idlib about the developments, and that the defeats in Idlib leave the major city of Aleppo as “the last hope for the Free Syrian Army.”

    The former military intelligence official said if the U.S. wants to support the Free Syrian Army there, “it must go after Assad” – whose forces have a large presence in Aleppo.

    "Aleppo is increasingly turning into a last stand, just like the Alamo,” the source said.

    Defense sources downplayed the reports of Nusra gains.

    One senior Defense official said they were “not tracking anything as big or catastrophic” as reported in the Post article.

    “There are ‘battles’ all the time between these groups, and we've yet to see anything I would characterize this way,” the source said. “Territory trades hands in local areas many times, and it is sometimes talked about in exaggerated terms to build propaganda."

    Earnest said he could not confirm the reports and said the administration is still assessing the situation.

    “We certainly are aware that moderate forces in Syria are engaged in a multi-front conflict, and that multi-front conflict is taking a toll on them,” he said. “There’s no doubt about that.”

    Earnest renewed calls to “ramp up” training and assistance to these forces.

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

    "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."

    Amid the advances by Nusra, 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.

    Nusra fighters are said to be gathering in the town of Sarmada in the northern Idlib province, a few miles from Bab al-Hawa.

    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. "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. – both are part of the Free Syrian Army alliance.

    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.

    Fox News’ Lucas Tomlinson and Jennifer Griffin and The Associated Press contributed to this report.