WASHINGTON/ISLAMABAD (Reuters) - The United States on Sunday stepped up demands that Afghanistan surrender Islamic militant Osama bin Laden, dismissing protests from the ruling Taliban that it has lost contact with the guerrilla leader who is now the most wanted man in the world. 
As mourners jammed New York's Yankee Stadium in a tribute to the nearly 7,000 people dead or missing in the Sept. 11 assaults on the World Trade Center and the Pentagon, U.S. forces massed within striking distance of the landlocked Central Asian country. 
Washington has bluntly warned the Taliban they will face U.S. military action unless they hand over bin Laden. 
"We are not going to be deterred by comments that he may be missing. We don't simply believe it," said Condoleezza Rice, the national security adviser for President Bush. "The Taliban is going to have to begin to understand it has a very tough choice to make." 
At the Camp David presidential retreat on Sunday, Bush and his wife, Laura, stood in an open field, their hands over their hearts, as Marines in dress uniform hoisted the U.S. flag to full staff for the first time since the Sept. 11 strikes. 
But prospects that the United States would soon return to normal looked dim as security and economic jitters spread. Crop-dusting planes were grounded across the country Sunday as the FBI urged vigilance for "suspicious activity" -- fanning fears of possible chemical or biological attack. 
In New York, the Mayor's Office said the number of those missing in the wreckage of the World Trade Center's twin towers had jumped to 6,453, up 120 from the prior toll amid continued checks of lists of those unaccounted for. 
Asian stock markets sagged in early trading on Monday as economic analysts warned of more heavy losses this week after war fears and dwindling consumer confidence sent U.S. blue chip stocks plunging last week to their worst weekly loss since the Great Depression. 
Secretary of State Colin Powell said the United States would release evidence linking bin Laden to the attacks, which occurred when teams of hijackers sent two commercial jets smashing into New York's World Trade Center and a third into the Pentagon outside Washington. A fourth hijacked jet crashed in Pennsylvania, apparently after passengers fought back against the hijackers. 
"I think in the near future, we will be able to put out a paper, a document, that will describe quite clearly the evidence that we have linking him to this attack," he said. 
Bush has demanded the Taliban hand over all leaders of bin Laden's al Qaeda organization in Afghanistan and close all camps used by the group or face military reprisals. 
In a sign it may be reconsidering its protection of the Saudi-born exile, a Taliban spokesman said the movement's spiritual leader had approved a request from the country's clerics that he leave the country. But he said the message was not yet in bin Laden's hands. 
"We have still not been able to deliver the clerics' message to him because we could not find him," Taliban spokesman Abul Hai Mutmaen told Reuters by telephone from the southern Afghan city of Kandahar. 
Washington scoffed at the Taliban statement. 
Asked if he believed the Taliban had lost track of bin Laden, Defense Secretary Donald Rumsfeld said: "Of course not. They know where he is." 
But Rumsfeld conceded that bin Laden -- who has denied involvement in the attacks -- may be able to evade U.S. forces, including special operations teams. "No, I'm not confident that we will find him and use this firepower," he said. 
Rumsfeld underscored the uncharted nature of Bush's new "war on terrorism" and the difficulty of defining victory in a conflict which, while triggering the biggest U.S. military mobilization since the 1991 Gulf War, remains without clear borders or targets. 
"This is not a quick effort, a battle, an event, television event with cruise missiles ending it, with a signing ceremony on the Missouri at the end of World War II. That isn't what this is about," he said. 
The FBI ordered all crop-dusting planes grounded until Monday morning, a move the Federal Aviation Administration said was made "for reasons of national security". Time magazine, in its issue due out Monday, reported that a manual for crop-dusting equipment had been found during a search of the suspected hide-outs of some of those believed responsible for the Sept. 11 attacks. 
In New York, an estimated 20,000 people flocked to historic Yankee Stadium on Sunday to hear prayers from the world's major religions and patriotic words from military leaders in tribute to those killed and missing in the attacks. 
Clutching flags and photographs of the missing, they joined "A Prayer for America" and sobbed for the devastation wrought when the World Trade Center's two 110-story towers collapsed following the hijack attacks. 
Bob Russell, a firefighter with Manhattan's Ladder Company 22, said he was there to pay tribute to some 30 colleagues lost at the trade center -- among a staggering total of 343 firefighters dead or missing in the disaster. 
"They were unbelievable those guys," Russell said, accompanied by his wife and children. "They're all gone." 
New York officials have said the chances of finding survivors 12 days after the attacks were very small, although rescue work continued. 
As the United States ramped up its response to the attacks, most Americans appeared solidly behind Bush, with 90 percent approving of the way he has handled his job, according to a CNN and USA Today poll. 
That was the highest rating for a U.S. president ever recorded by the Gallup polling group, overshadowing the previous record 89 percent scored by Bush's father, President George Bush, at the end of the 1991 Gulf War. 
In the world's markets, which fell sharply last week in response to the attacks and the possible U.S. response, investors were gearing up for more bad news on Monday. 
Stock markets in Sydney and Singapore dropped in early trading on Monday after last Friday's losses on Wall Street. Hong Kong's Hang Seng Index rose on a technical rebound, while the Tokyo stock market was closed for a holiday. 
U.S. military preparations picked up more steam. The Pentagon activated another 5,172 reserve troops on Saturday, a day after eight B-52 heavy bombers began departing for a buildup in the Gulf and Indian Ocean. 
U.S. officials said another deployment was expected to send additional warplanes to the area, bringing to more than 200 the number of planes that would join about 350 aircraft already in the region at land bases and on aircraft carriers. 
Bush was expected to issue an executive order designating specific groups and individuals as "terrorists" -- freezing their assets in the United States to starve them of financial support. 
Bush also moved to reward two allies, lifting U.S. sanctions imposed on Pakistan and India in 1998 for their nuclear tests. The move had been expected after Pakistan agreed to cooperate in the hunt for bin Laden despite stiff domestic opposition and substantial popular support for the Taliban. 
Gulf Arab states pledged support for the U.S. moves on Sunday while condemning any attempt to link Islam with what they termed "these heinous terrorist acts." 
Iran and Syria, meanwhile, said any reprisals for the attacks should be under the umbrella of the United Nations, while a senior Iranian cleric warned that Muslims would not stand by if a Muslim country were to be attacked. 
In Kabul, witnesses and officials said the Taliban was building bunkers, installing anti-aircraft batteries and arming men in key border areas to defend against any U.S. attack. 
Afghan opposition forces reported launching new attacks in three northern provinces and taking at least one district from the Taliban in a campaign timed to coincide with the threats of a U.S. military strike. 
