The US dropped a 15,000lb "daisy cutter" bomb on the entrance of a cave at Tora Bora in eastern Afghanistan yesterday in an attempt to flush out senior al-Qaida leadership, possibly including Osama bin Laden, thought to be hiding inside.

A spokesman, Rear Admiral John Stufflebeem, said the bomb, the third of its kind to be dropped in the course of the campaign, was intended to kill those inside and render the cave unusable.

He said the cave had been targeted because "it was believed that that's where some substantial al-Qaida forces would be, and possibly including senior leadership". The hunt for leaders of the Taliban was also stepped up in southern Afghanistan as US marines last night seized road junctions in an effort to seal off all escape routes around the deposed regime's former stronghold of Kandahar.

Hundreds of al-Qaida fighters were holed up in the Tora Bora caves yesterday, apparently undaunted by more than a week of heavy American bombing and repeated mojahedin attacks.

Arab fighters spread across the foothills of the White Mountains launched a barrage of mortar fire as 700 mojahedin troops loyal to three different commanders tried to begin a new, coordinated guerrilla offensive.

Pashtun soldiers have now been pushed back from the valley of Melawa, which they won last week, and were yesterday forced to defend themselves with anti-aircraft and machine guns and tank fire from their rear positions.

Afghan commanders admit they expect little rapid progress in the battle against al-Qaida. Their fighters have been sent out with seven days' food but most carry just two spare magazines of ammunition for their Kalashnikovs.

"They have as much ammunition as we think is needed. It is a war. We don't know whether we will defeat them or be defeated ourselves," said Hafta Gul, a commander coordinating the attack from a disused veterinary office near the frontline.

Cdr Gul said the Arab forces had held several meetings in the past week to discuss battle plans. Villagers had been instructed to take messages across the frontline to the mojahedin.

"The Arabs asked the villagers to tell the mojahedin that they had decided not to leave Tora Bora until they are killed," Cdr Gul said. Arab fighters also sent a letter imploring the mojahedin to join them in a battle against US forces. "They said they had come here to be martyrs."

The goal of yesterday's three-pronged ground offensive, which began at lunchtime, was to recapture the valley of Melawa. Small 10-man teams were sent up into the mountains hoping to surround the Arab positions.

Many of those who have been sent on to the wooded ridgelines over the past week are suffering from the cold and lack of food. "It took us four days before we could send blankets and equipment up to them. You can see how their morale will be affected," said Haji Zahir, one of the three commanders leading yesterday's attack.

Mojahedin commanders say Osama bin Laden is near the Tora Bora caves, although the last reported sighting of him was at the end of last week. "If luck is with him he can escape, otherwise he will be taken by our forces," Cdr Zahir said.

In Kandahar, US forces were reported to be preparing to screen all those attempting to leave the city and the wider area as part of their dragnet for those on America's list of wanted terrorists.

The US deputy defence secretary, Paul Wolfowitz, said that "two or three" senior Taliban leaders had been captured in recent days. US sources said one of them was the Taliban's army chief of staff.

Travellers arriving at the Pakistan border town of Chaman said they had seen about 30 US tanks and other armoured vehicles moving towards Kandahar yesterday. One said a US serviceman had been seen on the roof of the Taliban's spiritual leader, Mullah Mohammed Omar's compound, although that could not be independently confirmed.

Although the whereabouts of Mullah Omar remains unknown, witnesses say that he was last seen on Thursday in the Sinzeri area, not far from the city.

The movement of marines to set up roadblocks around the city was confirmed by a spokesman, Captain Stewart Upton. He said the forces were not moving into Kandahar itself but were "reinforcing current operations".

"We are continuing to look for the al-Qaida and Taliban who still have their weapons," Cpt Upton said. "If the Taliban hold their weapons, they will die."

Another spokesman, Captain David Romley, acknowledged the dangers the marines face as they move closer to the city, which Pashtun forces took on Friday as Taliban troops fled with their weapons. Other Taliban fighters are believed to remain in Kandahar. "The closer you get to Kandahar the more dangerous it gets," he said.

The marines are carrying photographs of known terrorists with them.

In Kandahar, Afghanistan's interim leader Hamid Karzai said he intended to quickly disarm factions in the city, which was yesterday reported to be largely quiet with only sporadic gunfire following two days of violence.