WASHINGTON - U.S. ground forces are operating in northern and southern Afghanistan in a secretive phase of the war on terrorism, and additional troops are poised for commando raids in search of Osama bin Laden and his al-Qaida lieutenants, officials said Friday.

The arrival of small numbers of U.S. troops on the ground in Afghanistan does not indicate an intention to launch a conventional ground offensive but rather a shift to a broader range of military activities _ both overt and covert _ after nearly two weeks of aerial bombardment, U.S. officials said.

The bombing resumed early Saturday in Kabul, with at least eight explosions rattling the city from the northeast.

Special operations troops such as the Army's Green Berets perform many different missions, including assistance to opposition forces and collection of intelligence. Special forces in southern Afghanistan are supporting the CIA's effort to encourage ethnic Pashtun leaders to break away from the Taliban militia, a U.S. official said Friday, speaking on condition of anonymity.

Defense Secretary Donald H. Rumsfeld declined to comment on that.

Rumsfeld did say that the United States was coordinating with rebels in northern Afghanistan as well as other anti-Taliban forces.

"There is good coordination from the air with the ground in some places, particularly in the north," Rumsfeld told reporters who flew with him Friday to Whiteman Air Force Base, Mo., to visit with crews of B-2 stealth bombers. "There is not that kind of coordination as of yet in the south."

In a later session with reporters at Whiteman, Rumsfeld was asked whether the war against terrorism would have to be fought in countries outside Afghanistan in order to be successful.

"There's no doubt in my mind," he replied.

Separately, U.S. officials said the air assaults that began Oct. 7 and continued Friday will intensify soon and focus more directly on front-line troops of the ruling Taliban.

Afghan forces opposing the Taliban are attempting to take advantage of the bombing and are hoping for additional U.S. military assistance. Rumsfeld said Friday that the opposition forces known as the northern alliance have asked for and received U.S. aid, including ammunition or money to buy it.

Gen. Rashid Dostum, an ethnic Uzbek warlord who is a northern alliance commander, told The Associated Press on Friday his forces have been holding talks with several U.S. military personnel this week near the city of Mazar-e-Sharif, just south of the border with Uzbekistan, where U.S. forces are stationed.

Dostum said his discussions with the Americans centered on delivering humanitarian aid to Dara-e-Suf, an enclave of opposition resistance south of Mazar-e-Sharif.

U.S. Air Force C-17 cargo planes have dropped more than 500,000 ration packages in Afghanistan, but U.S. officials are hoping to make overland deliveries, which could get more aid to the needy more quickly.

In Islamabad, a Pakistan military official said Friday that American officials informed his government that U.S. special forces will be conducting "hit-and-run" operations in Taliban-ruled areas of Afghanistan in an effort to flush out bin Laden, members of his al-Qaida network and Taliban leaders.

An unspecified number of U.S. special forces were dropped into southern Afghanistan on Thursday, the official said on condition of anonymity. The official said Pakistan was told U.S. forces have been in northern Afghanistan for more than one week.

Bush administration officials have said repeatedly since the U.S.-led military action began that special operations forces would play an important role in Afghanistan. But because surprise and deception are critical, officials have declined to discuss specifics.

"If or when they are on the ground, being there would make them the most vulnerable individuals engaged in this campaign," Rear Adm. John Stufflebeem, deputy director of operations for the Joint Chiefs of Staff, said Friday.

Asked what special operations forces might bring to the particular problem of fighting terrorists in Afghanistan, Stufflebeem mentioned their expertise in developing intelligence information, training opposition forces and conducting direct action such as quick-strike offensive raids.