WASHINGTON/KABUL (Reuters) - U.S fears of unconventional terrorist attacks spread from anthrax and germ warfare to nuclear safety on Thursday when its most famous nuclear power plant was put on top alert as U.S. planes pounded Afghanistan for the 12th day in their war on terrorism. 
Officials later dismissed the threat to Three Mile Island after first calling it "credible," but the incident at the site of the worst nuclear accident in U.S. history jangled nerves already frayed by last month's attacks on New York and Washington and three dozen subsequent cases of anthrax or exposure to the potential germ warfare weapon. 
On the military front, U.S. planes in Afghanistan continued a fierce bombardment of targets of the ruling Taliban, who are sheltering Osama bin Laden, the prime suspect in the Sept. 11 attacks by hijacked airliners that killed nearly 5,400 people. 
A senior Taliban spokesman said between 600 and 900 people had so far been killed, or were missing, in the U.S. strikes. 
On the diplomatic front, President Bush arrived in China for an Asian summit, hoping to shore up support from countries as diverse as China, the world's largest remaining communist country, and Indonesia, the most populous Muslim nation. 
But it was the home front with confirmed cases of the potentially deadly anthrax now standing at six, including a postal worker and an assistant at a third major TV network, as well as the nuclear scare that preoccupied Americans. 
Harrisburg International Airport and Lancaster airport near Three Mile Island nuclear plant in Pennsylvania were shut for four hours. The plant itself, which was site of the worst U.S nuclear accident in 1979 although there were no casualties, was on top alert for about eight hours. 
"We took the appropriate actions putting the reactor on a heightened state of alert last night following a report from the federal intelligence community of a threat to Three Mile Island," Neil Sheehan, a spokesman for the Nuclear Regulatory Commission (NRC), said. 
"We no longer consider that threat credible," Sheehan said, adding it was "very specific" to the Three Mile Island facility. He declined to elaborate. 
Equipment failure at the plant in 1979 caused a loss of cooling water, leading to the partial melting of the uranium fuel and release of a small amount of radioactive material. 
Any attack whether by a hijacked plane or other means on the plant, which has a heavily reinforced containment shell, would presumably seek to release deadly radioactivity into the air. 
Local television stations said temporary flight curbs were put into effect for a 20-mile radius around the airports and military aircraft were sent to protect the plant. 
All 103 U.S. nuclear power reactors, among the most closely guarded facilities in the country, have been on even higher alert after the Sept. 11 attacks on New York's World Trade Center and the Pentagon. 
Although no hard evidence has been found linking anthrax-laced letters to bin Laden, Bush has said there could be a link. 
The government offered a $1 million reward for information leading to the arrest and conviction of those responsible for mailing anthrax, FBI Director Robert Mueller announced. 
The spread of anthrax contamination by letter forced an unprecedented closing of part of U.S. Congress for an environmental safety check on Wednesday and spooked financial markets around the world. 
The Republican-controlled U.S. House of Representatives and all Capitol Hill office buildings closed until Tuesday for anthrax testing, but the Democratic-controlled Senate held a short session under heavy security after 31 congressional staffers tested positive for anthrax exposure. 
The contamination arrived in an anthrax-tainted letter at the office of senate Majority Leader Tom Daschle on Monday. 
The last time there was such a closure was on the day of the September attacks when the Senate also shut down. 
Despite the disruptions of the anthrax scare, congressional leaders said they had reached a compromise agreement on a broad anti-terrorism bill to expand the power of law enforcement to wiretap suspected terrorists, share intelligence information about them and track their Internet movements. 
The two latest cases were confirmed on Thursday. 
A CBS employee who works with TV News anchor Dan Rather in New York tested positive for skin anthrax, making CBS the third major network to be exposed to the disease after NBC and ABC. 
In New Jersey, where two of the anthrax-containing letters were postmarked, a postal worker also tested positive. 
The U.S. Postal Service is taking the extraordinary step of sending notices to every home and office instructing Americans on how to identify and handle suspicious letters and packages. 
But the scare rages on. More than 60 people in Burlington, Vermont, began antibiotics treatment as a precaution after a powdery substance officials said was not likely anthrax was found on an arriving jet. 
The anthrax scare has spread far beyond U.S. borders. 
Security guards sealed off the mailroom of the lower house of the French parliament, and part of Australia's national parliament was evacuated after powder-laced letters arrived. 
In Nairobi, a letter sent to an unidentified Kenyan businessman tested positive for anthrax. The letter was posted from Atlanta on Sept. 8 and passed via Miami. 
In Japan, letters containing suspicious powder were delivered to the U.S. consulate in Osaka and three major Japanese newspapers. 
In Afghanistan, the 12th day of air raids went ahead despite appeals from aid agencies for a break to get badly needed food into the country. 
For the first time ever, the United States is flying armed, unmanned drones into combat. "Predator" spy planes, armed with anti-tank missiles are taking to the skies over Afghanistan, U.S. defense officials said on Thursday. 
The remote-controlled RQ-1 aircraft have been modified by the Air Force to carry two Hellfire missiles. Defense experts called such a move a first step toward perhaps one day building unmanned, long-range bombers that can carry dozens of missiles and bombs to overseas targets without risking human crews. 
Senior Taliban spokesman Abdul Hai Mutmaen said between 600 and 900 people had so far been killed or were missing. 
"And don't ask me about the number of wounded, because it is in the thousands and I don't have a figure for that," Mutmaen told Qatar's Jazeera television in a videophone interview from an unidentified location. 
Thursday's morning raids hit targets around Kandahar and Jalalabad in the east -- the hub of Afghanistan's notorious guerrilla training camps, witnesses told Reuters. 
At least seven civilians were killed and several injured by exploding ammunition after U.S. warplanes bombed a Taliban munitions dump the north of the capital Kabul, witnesses said. 
But the Taliban, who have denounced the air strikes as a war on Islam, said all their leaders were alive and well and so was their guest bin Laden. 
The military chief of bin Laden's al Qaeda network said Afghans would drag slain U.S. troops through the streets, rekindling memories of the doomed 1993 involvement in Somalia when 18 U.S. members of a U.N. peacekeeping force were killed and mobs dragged some of their bodies through the streets. "America will only be certain about its mistaken calculations after its soldiers are dragged in Afghanistan as they were in Somalia," Egyptian radical Mohamed Atef, reportedly bin Laden's No. 2, was quoted as saying in a report by the London-based Islamic Observation Center. 
In New York, four followers of bin Laden were sentenced to life in prison without parole for the 1998 bombings of two U.S. embassies in Africa, the first U.S. convictions linked to the Saudi-born militant. 
With Bush keen to ensure global backing for his war on terrorism, latest developments in the Middle East threatened to complicate his task of selling the idea to skeptical Muslims. 
Israel, incensed at Wednesday's assassination of right-wing cabinet minister and former general Rehavam Zeevi, threatened to invoke the war on terrorism to justify attacking the Palestinians if they fail to hand over Zeevi's killers. 
Any flare-up in the Middle East could alienate the very Muslim nations Bush is courting at his meetings in China.