The men, whose identities were not revealed, were taken prisoner by the Pakistani army following tip-offs by tribal chiefs, sources said.
"Yes, we confirm it," Zamir Akram, deputy chief of mission at the Pakistan Embassy in Washington, said when asked about the arrests. "It is an ongoing operation.
"We have been arresting people and handing them over to the Americans," he said.
It was not known if the five or six men were already in the United States. The definition of "American origin" was also unclear.
The FBI, when asked about the handover, referred all questions to the Justice Department. The Justice Department was not immediately available for comment.
U.S. Attorney General John Ashcroft, together with FBI and Department of Defense officials, announced Monday the arrest and detention of New York-born Jose Padilla. Padilla, who allegedly received bomb-making training from al Qaida and was part of a conspiracy to plant a radiological bomb somewhere in the United States.
Al Qaida, the terror network of Osama bin Laden, is accused of the Sept. 11 attacks on New York and Washington that killed about 3,000 people, and of earlier terror strikes, including the bombing of the USS Cole in Yemen and the bombing of U.S. embassies in Kenya and Tanzania
Padilla, who adopted the name Abdullah al Muhajir, was arrested at Chicago's O'Hare International Airport on May 8 as he returned to the United States from Pakistan in a helter-skelter route that took him twice to Zurich, Switzerland, and once to Egypt.
U.S. officials Tuesday said Padilla was shadowed on his odyssey to Chicago by U.S. law enforcement and intelligence officials who even traveled on the same aircraft with him.
A Pakistani security official, speaking by telephone from Islamabad, told UPI "Padilla went to Europe where he met several of his contacts while FBI agents were watching him. They might have made some arrests in Europe too, but we don't know."
News reports indicate the journey took place over 1 1/2 weeks, but a U.S. intelligence source told UPI Tuesday it "was closer to a month."
"We have captured a known terrorist who was exploring a plan to build and explode a radiological dispersion device, or 'dirty bomb' in the United States," Ashcroft said Monday.
"He traveled to Afghanistan and Pakistan and on several occasions in 2001, he met with senior al Qaida officials. While in Afghanistan and Pakistan, Padilla trained with the enemy, including studying how to wire explosive devices and researching radiological dispersion devices."
Officials said Padilla, who is believed to have visited Switzerland to get money for his operation, was coming to the United States on a reconnaissance mission, scouting out possible targets for the conventional bomb that would spew radioactive materials into the air.
Pakistani officials said they tipped the United States off to Padilla.
U.S. authorities said his alleged connection to al Qaida was gained through information provided by captured al Qaida operations chief Abu Zubaydah and other al Qaida captives and a checking of an FBI database.
Padilla, 31, is an ex-convict and street gang member who grew up in Chicago, converting to Islam while in prison.
He is believed to have traveled to Pakistan and Afghanistan in 1998.
Although news of his arrest and detention was only released Monday, it is believed members of the Senate and House Select Intelligence Committees were informed earlier.
"I understand the intelligence committees were given information about Padilla within the last several weeks," a U.S. government source said on condition of anonymity, "but I don't know how much detail they went into."
A Western European diplomat told UPI his embassy had been informed of Padilla's capture last week.
Padilla, being held as an "enemy combatant," is the third American detained in connection with al Qaida and its Afghan protectors, the extremist Taliban regime deposed late last year by U.S.-backed Afghan forces.
John Walker Lindh, 20 at the time of his capture in Afghanistan by anti-Taliban forces last December, faces trial in Alexandria, Va., on charges ranging from providing support to a terrorist organization to conspiracy to kill U.S. citizens abroad.
Yasser Esam Hamdi, 22, is being held at a U.S. Navy brig in Norfolk, Va., but has not yet been charged with any offense. Hamdi, born in Louisiana but raised in Saudi Arabia, was also captured fighting with the Taliban.
A senior administration official, speaking on condition of anonymity, said Monday that no formal charges had yet been lodged against Padilla.
U.S. Secretary of Defense Donald Rumsfeld, currently in the Persian Gulf, said Tuesday Padilla may not be charged.
"... Our interest in his case is now law enforcement," Rumsfeld said, after noting Padilla could possess valuable information. "It is not punishment because he was a terrorist or working with terrorists.
"Our interest at the moment is to try to find out everything he knows so hopefully we can stop other terrorist acts.
"We're not interested in trying him at the moment. We're not interested in punishing him at the moment. We're interested in finding out what in the world he knows, " Rumsfeld said.
The holding of a U.S. citizen as an enemy combatant is allowed under law and Supreme Court precedent made in 1942 in regard to two U.S. citizens engaging in sabotage for Nazi Germany.
Padilla was detained on his arrival in Chicago under a material witness warrant and sent to a federal facility in New York, where he was questioned. He reportedly had more than $10,000 in cash on him at the time of his arrest.
Reports said he was uncooperative, and the decision was made to transfer him to military custody as an "enemy combatant," since it would allow authorities to hold him longer and under tighter control.
Padilla was sent Sunday to the Naval Consolidated Brig in Charleston, S.C.
The facility, a Level II medium security prison with special high security capacity, currently holds 260 prisoners from all U.S. military service branches.
It has 400 cells and is considered one of the top high-tech U.S. military detention facilities.
Pakistani and U.S. sources said Padilla was detained by the Pakistani's in early April over an immigration matter, but then released. The two Pakistanis detained with him are said to still be in custody.
Pakistani security sources said U.S. officials were told of his detention and he was let go at their behest and then followed.
U.S. officials Tuesday could not confirm they requested Padilla's release, but did say he was closely monitored by U.S. and local authorities from then on as he traveled from Pakistan to Zurich, to Egypt, back to Zurich and then to Chicago.
"It was a combination (of U.S. and local police)," said an American official who requested anonymity. "We had eyes on him the entire time.
"Our guys had him in their sites always."
A Pakistani intelligence official said Monday the operation to capture al Qaida and Taliban remnants in Pakistan's tribal areas, to which they fled from Afghanistan to escape U.S. and coalition forces, would continue.
He also heralded the capture and operation as a sign of Pakistan's cooperation with the United States in combating terrorism.
"We're doing much more," he said. "Some things will be known, much more will never be known."
Asad Hayauddin, the Pakistani Embassy spokesman in Washington, added: "We have been working with the Americans ever since the war on terror began. It's their equipment and our men."