KANDAHAR, Afghanistan — Afghan President Hamid Karzai returned Sunday to a Taliban stronghold where he was nearly assassinated 19 months ago, and said he would welcome rank-and-file members of the militia back into society.

But Karzai said about 150 leaders of the ultra-religious Islamic movement supplanted by his government after a U.S.-led war are unworthy of rehabilitation and could be prosecuted.

"Our problem is mainly with the top Taliban -- who may number no more than 150 people -- who had links with al Qaeda," Karzai said. "Those people are the enemies of Afghanistan and we are against them."

"But those Taliban who are doing jobs and tilling the fields and working as shopkeepers, we want to welcome those Taliban," he said.

The president has said in the past that he believed most Taliban could be reintegrated back into society, but this appeared to be the first time he put a number on those the nation sought to prosecute.

Karzai said the government has been in negotiations with less radical Taliban leaders for months, though he did not give any details or names. Taliban leader Mullah Mohammed Omar and his inner circle would not be expected to be offered an olive branch by the government.

The idea of reintegrating less radical elements of the Taliban has been floated for months, and there have been reports the government is also negotiating with some supporters of renegade warlord Gulbuddin Hekmatyar in an effort to get them to switch sides.

The notion appears to have Washington's support.

On Tuesday, U.S. ambassador Zalmay Khalilzad said he favors an amnesty for all but the worst of the old regime -- those that allied themselves with Osama bin Laden's al Qaeda network and committed crimes against humanity.

It was Karzai's first trip to Kandahar since a gunman opened fire on him in September 2002 outside the provincial governor's house. The gunman and two other people were killed when U.S. special forces protecting Karzai fired back.

Security was tight Sunday, and a local security official said at least one man was arrested in Kandahar on suspicion of plotting terrorist acts.

The Taliban have stepped up attacks in recent weeks, especially in Kandahar and other southern and eastern provinces.

Spc. Pat Tillman, a former NFL player with the Arizona Cardinals, became the latest U.S. casualty when he was killed in an ambush Thursday by suspected Taliban militants in the eastern province of Khost. Several bombings have been reported in Kandahar in recent weeks, one outside a building where the governor was holding a meeting.

A two-car UN convoy carrying national and international staffers working on preparations for September elections was targeted in a bombing attack on a road last week just outside Kandahar, UN spokesman David Singh said at a press conference in the capital on Sunday. There were no injuries, but one of the vehicles was damaged and thrown back several yards after the bomb went off on Thursday morning.

A man believed to be trying to plant a bomb at another point on the same road blew himself up earlier that morning.

"All UN operations in the region have been temporarily suspended," Singh said. "That includes voter registration sites which have been closed pending a security assessment."

Singh said a warehouse in the northern city of Kunduz was also damaged Thursday when it was struck by artillery. There were no injuries.

The United Nations has warned that if security throughout the country does not improve, it will be nearly impossible to hold successful elections.

The vote was to take place in June, but violence has slowed voter registration, forcing the delay.

On his Kandahar trip, Karzai met with the governor and with tribal elders. He urged residents to participate in the elections by registering to vote, and called on militiamen under the control of local warlords to embrace the government's disarmament program, which provides economic benefits for those that lay down their arms.

Karzai also traveled to his hometown of Karz, just outside Kandahar, and visited his father's grave.

The government has said it hopes to disarm 40 percent of the estimated 100,000 private militiamen throughout the country by the end of June, though most observers say that goal is overly optimistic.

Many warlords are nominally allied to Karzai's government, and the administration relies on their muscle to keep order in the lawless provinces.