People carry the remains of a victim killed in an explosion at a bullet factory in the southern Yemeni town of Jaar. (STR/REUTERS)

An explosion ripped through crowds of looters in a munitions factory Monday, killing at least 78 and injuring scores in the latest sign of weakening government authority amid Yemen’s two-month-old populist uprising.

The accidental blast came after soldiers abandoned the factory, allowing the looters to enter — one of a series of incidents in recent days in which government forces have left their posts. They include areas in the north, where rebels have long challenged the state’s authority, and the south, where al-Qaeda’s Yemen branch and secessionists have confronted government forces for years, according to Yemeni and opposition officials, diplomats and local news reports.

The spreading instability underscores growing concerns by the United States and its allies that this fragile but strategic Middle Eastern nation could collapse if its political tensions are not peacefully resolved. It comes as high-level negotiations between President Ali Abdullah Saleh and his opponents over a handover of power have stalled in recent days. Saleh declared that he would give no more concessions to the opposition.

He has also publicly warned that Yemen could fall into civil war if he is forced to leave office prematurely. In an interview Sunday with al-Arabia television network, he described Yemen as “a time bomb” and suggested that it could possibly end up as chaotic as Somalia. Yemen, he said, he could end up divided “in four parts.”

In Washington, an Obama administration official said Tuesday that the “protracted standoff” in Yemen between Saleh and anti-government protesters “has had a direct adverse impact on the security situation throughout the country.”

The official said various groups, including al-Qaeda, Shiite Houthi rebels, tribal elements and secessionists “are exploiting the current political turbulence and emerging fissures within the military and security services for their own gain.” He warned that until Saleh resolves the current impasse “by announcing how and when he will follow through on his earlier commitment to take tangible steps to meet opposition demands, the security situation in Yemen is at risk of further deterioration.”

The tensions have grown worse since March 18, when snipers loyal to Saleh fired into crowds of anti-government protesters, killing at least 52. That triggered a wave of defections by top military officials, tribal leaders, diplomats and ministers.

Security forces have grown dramatically in the capital of Sanaa and in other cities, suggesting that they have been brought from other areas of the country.

In interviews, opposition leaders cynically claimed that Saleh, one of the Middle East’s wiliest politicians, was manipulating the situation for his own political gain. Security forces, they contended, are not abandoning their positions, but rather are being systematically pulled out to foment chaos and make Saleh feel irreplaceable.

“This is the government’s strategy to convey the message to the international community that if Saleh leaves, he will be replaced by terrorist groups. This is their last card,” said Yassin Noman, head of Yemen’s opposition coalition.

Still, Western diplomats and analysts say there are indications that al-Qaeda, the northern rebels and southern secessionists appear to be taking advantage of the political crisis and solidifying their positions.

In the northern province of Saada, Houthi rebels entered the provincial capital last week, forcing the governor to flee. A committee comprised of the rebels, residents and defected military commanders has appointed Faris Manna, a reputed arms dealer, as the new governor, according to Yemeni officials, opposition leaders and Western diplomats. Government officials have also been driven out of Jawf, east of Saada.

In Marib province, al-Qaeda militants killed seven soldiers in an attack Sunday, the latest in a series of recent attacks on security forces. In the south, Islamist militants seized the towns of Jaar and al-Husn in Abyan province, a known al-Qaeda haven, according to local news reports. The munitions factory in Monday’s explosion was about three miles west of Jaar.

One witness told the Associated Press that the militants took two armored cars, a tank, several pickup trucks mounted with machine guns and ammunition. Later, large groups of impoverished people entered the factory to loot anything of value.

“This accident is a true catastrophe, the first of its kind in Abyan,” said a doctor at the state-run hospital in Jaar, according to Reuters. “There are so many burned bodies. I can’t even describe the situation.”

The deputy governor of Abyan province, Saleh al-Samty, blamed the government for the explosion, saying it was because of the pullback of security forces and resulting disorder, according to the Associated Press.