WASHINGTON (AFP) - US warplanes on yesterday bombed a compound southeast of Kandahar believed to contain senior al-Qaeda and Taliban leaders, US Defense Secretary Donald Rumsfeld said.

"Those who were there will wish they weren't," Rumsfeld said on the flight back to Washington after watching the attack live from the headquarters of the US Central Command in Florida.

He said he had no names of the leaders believed to be in the targeted facility. but said they were of "appreciable importance.

"It clearly was a leadership area," he said. "All of the indicators demonstrated it was a nontrivial leadership activity."

Earlier yesterday, Rumsfeld warned that the US campaign in Afghanistan was entering a difficult and dangerous phase as US forces shift to hunting down leaders and fighters of terror suspect Osama bin Laden's al-Qaeda network.

Rumsfeld said fighting was still underway in a fortress prison in the northern Afghan city of Mazar-i-Sharif where dozens of armed Taliban and al-Qaeda followers held out for a third day against US air strikes and Northern Alliance forces on the ground.

"It is a difficult environment for the Americans that are there, difficult environment for the coalition forces that are there, and a difficult environment for the opposition forces who are attempting to provide some stability in those villages and towns," he said.

Rumsfeld was in Tampa to visit the headquarters of the US Central Command, whose chief, Army General Tommy Franks, is commanding the 50-day-old US campaign.

His visit comes on the heels of the insertion of hundreds of US marines into an airfield some 80 miles south of Kandahar, the Taliban's last bastion and the headquarters of its leader Mullah Mohammad Omar.

Tribal forces

Within hours of their arrival, Marine Corps AH-1W helicopter gunships took part in an attack on an armored column, disabling a number of tanks and other pieces of heavy equipment in the vicinity of the airstrip.

"Their very presence does in fact provide pressure, but I would not characterize the intent of them being there as a force to attack Kandahar. That simply is not the case," said Franks, who joined Rumsfeld at a news conference here.

Tribal forces were pressing in on the city from both the north and the south, he said.

"Kandahar is a very confused place right now," Franks said. "We see evidence that a great many people of the non-Afghan type are working very hard to get out of Kandahar," he said.

He said US air forces were taking care to bomb the area selectively, avoiding what he called "high value collateral damage targets."

Rumsfeld said US forces would be shifting their emphasis from the cities to the hunt for the leaders of al-Qaeda, the network suspected of masterminding carrying out the Sept 11 terrorist attacks on the United States.

"We'll pursue them until they have nowhere else to run. Let there be no doubt this campaign is far from over," Rumsfeld said.

"Our efforts, of course, will be shifting at some point from cities to hunting down, and rooting out terrorists where they hide," he said.

Network of caves

Franks said intelligence has focused the US search on two areas - one in the vicinity of Kandahar and the other an area from Kabul east to the Khyber pass, including Jalalabad and Tora Bora.

Asked whether US forces or their allies have begun to search the vast networks of caves and tunnels believed to be used as al-Qaeda and Taliban hiding places, Franks would say only that the United States had forces on the ground.

Rumsfeld said Pakistani forces were monitoring some 170 mountain passes in the border area to block the movement of al-Qaeda or Taliban forces.

The defense secretary said also that prisoners were being interrogated and tips were pouring in as a result of a 25-million-dollar bounty put on the head of bin Laden, the al-Qaeda leader.

US forces also have identified more than 40 places that may have been used for research and development of weapons of mass destruction and were in the process of systematically collecting and analyzing samples taken from them, Franks said.

"We have not found something that we believe is a specific thing," he said.