Suspected rebels detonated a bomb beneath a crowded bus crossing a wooden bridge in Nepal's (search) rural south Monday, officials said, killing at least 38 people and wounding the other 71 passengers on board in one of the bloodiest attacks against civilians since an insurgency broke out nearly a decade ago.

An initial investigation showed a rebel hiding behind a tree used an 820-foot-long wire to remotely trigger the bomb, army officials said.

The army said it was certain the explosion was detonated by rebels fighting since 1996 to abolish Nepal's constitutional monarchy and set up a communist state. The attack came with no warning in an area many believed to be relatively safe from insurgent attacks. It was not known why the bus was targeted.

The rebels did not comment on the explosion and remain out of reach in their mountain bases.

The guerrillas have stepped up violence since Feb. 1, when King Gyanendra (search) took control of the government and imposed a state of emergency subsequently lifted in April. The king said his February power grab was necessary to quell the communist insurgency, which has left more than 11,500 people dead.

The rebels, who claim to be inspired by Chinese revolutionary Mao Zedong (search), responded to the king's actions by shutting down highways and calling a general strike. They have repeatedly refused government calls for peace talks.

Government troops moved in after Monday's blast, searching for rebels and taking control of the area, 110 miles southwest of the capital, Katmandu.

The bus was traveling on a rural highway near the village of Badarmude when it was ripped apart by the explosion, killing 38 people and wounding 71, an army official said on condition of anonymity because of military policy. Everybody on board was either killed or wounded.

"There was a small bang and then our bus was thrown in the air. The bus was ripped into pieces and many people were killed," said Khum Bahadur Gurung, 62, who suffered leg injuries and spoke to The Associated Press from his hospital bed.

The bus packed with passengers was crossing a wooden bridge when the explosion went off. It was thrown up in the air and landed beside the highway on the banks of the Mude river. Gurung said parts of the bus were scattered across the river bank, and many of the bodies were charred.

In New York, U.N. Secretary-General Kofi Annan condemned the "senseless deaths" of civilians and called for an immediate end to such attacks. Annan expressed his condolences to the families of those killed or wounded in the blast via a statement from a spokesman.

Some of the injured were flown by army helicopters to Katmandu and were being treated at the army hospital.

Buses traveling in the poor area usually are crowded, with most passengers standing in the aisles or even holding onto the roof. The rural highway and the bridge were covered with gravel, making it easy for the rebels to hide the land mine.

Although the rebels have repeatedly said they do not target civilians, they have attacked civilian passenger buses defying their calls for transportation strikes. But the rebels have called no strike or blockade in the area of the bombing this week.

Officials said three off-duty soldiers were on the bus but they were unarmed and returning to duty.