Troops loyal to Syrian President Bashar Assad have broken through rebel forces encircling the northern city of Aleppo to secure a major chemical weapons base that, if they can hold it, will be a big help to U.N. experts sent in to destroy Assad's chemical arsenal.

The sprawling facility at al-Safira is one of the most important chemical warfare centers in Syria, and has been one of the most threatened by the rebels, including hard-line jihadist groups like the al-Nusra Front.

The nightmare scenario for Israeli and Western leaders is that the jihadists get their hands on Assad's vast armory of chemical weapons and deadly nerve agents like sarin and VX.

The capture of al-Safira, which includes a heavily guarded facility where nerve agents are produced and weaponized, could allow the U.N. specialists to eliminate a significant portion of Assad's weapons of mass destruction.

The 100 U.N. specialists already in Syria, or scheduled to arrive over the next few weeks, for the eight-month mission, will need all the help they can get in their year of living dangerously.

U.N. Secretary-General Ban Ki-moon has said they face "unprecedented dangers" in seeking to dismantle Syria's estimated 1,000-ton chemical arsenal in the middle of a 2 1/2-year-old civil war under extreme deadline pressure.

That makes their mission, which Ban described as an "operation the likes of which has never been tried before," the most hazardous ever undertaken in the history of disarmament.

Assad's forces, supported by mobile artillery and airstrikes, Monday broke through rebel lines to secure the chemical weapons facility at al-Safira, where important defense plants are located, outside the strategic city of Aleppo.

The city, once the commercial heart of Syria, has been the center of a major battle since early August when Assad's forces, bolstered by fighters from Lebanon's Hezbollah, launched a big offensive in the north.

The regime push that led to Monday's capture of Al-Safira swelled last week, although it's not clear whether that was linked to Assad's decision to surrender his chemical weapons in return for the Americans setting aside plans for missile strikes against key regime targets.

But on Oct. 3, the regime retook the key town of Khanasser near Aleppo which controls the main highway to al-Safira, allowing Assad's forces to break through to Al-Safira four days later.

"This development will be welcomed even by the United States, whose opposition to the Assad regime is currently outweighed by its concerns for securing the chemical weapons in Syria," the U.S. intelligence consulting firm Stratfor said.

"It's not clear if the regime will be able to adequately secure the al-Safira defense plants and the roads leading to it, but if they do, one of the greatest constraints to the chemical weapons cleanup will have been removed."

At al-Safira, set in the rolling hills outside Aleppo, the regime has long operated a large plant for the production of sarin, the nerve agent believed used by the regime in the Aug. 21 attack on the outskirts of Damascus that killed, by U.S. count, more than 1,400 men, women and children, and triggered the global outcry.

Fighting has swirled around al-Safira for much of the war, since it's a highly prized target for the rebels. But the nerve gas plant has remained firmly under Syrian army control.

In late 2012, the Syrian army sent a large, heavily armored relief column into al-Safira, possibly to remove some of the stockpile of sarin and other nerve agents stored there to more secure bases.

Al-Safira was expanded in 1987 and 1995 as Assad's father, Hafez Assad who died in 2000, built up what would become the third largest chemical weapons arsenal in the world after the United States and Russia.

The elder Assad, a Soviet-trained fighter pilot who seized power in a November 1970 military coup, was a close ally of Moscow and started building his chemical arsenal in the late 1970s with Soviet help.

Al-Safira also houses a military storage area on its eastern side, a complex of igloo-shaped bunkers where chemical weapons are presumably held.

On the western side, there are larger rectangular buildings that experts say are likely storage facilities for Soviet-designed Scud ballistic missile and chemical warheads.