WASHINGTON/KABUL (REUTERS) — U.S. warplanes stepped up their bombardment of Afghanistan Wednesday as the country’s reclusive Islamic leader called for help from the Muslim world but seemed unlikely to get much official aid.

A U.S. official who asked not to be named said two of Taliban leader Mullah Mohammad Omar’s close relatives were killed during the first day of U.S. and British strikes on Afghanistan Sunday. The offensive, launched in response to last month’s mass killing of Americans and other nationals, aims to root out terrorism and governments that support it.

“There were two close relatives of his, adult males, who were killed,” the official told Reuters. He said other Taliban military officers had also been killed in the strikes.

Military officials declined to say whether the United States was using its massive “bunker-busting” bombs — 5,000-pound laser-guided weapons that were used against Iraq to devastating effect in the 1991 Gulf War.

But residents of Kabul said the bombing was the heaviest yet. Anti-aircraft fire blazed away for much of Wednesday night, and jets screamed over the capital as bombs and missiles struck at targets around the city, including the airport and near a central residential area.

A huge fire blazed for a while near a lubricant storage area of the airport to the north of the city.

“This is the worst night that we have had so far,” said one resident. “There has been no chance to sleep. I cannot tell you how frightened people are. It is terrible.”‘

President Bush said “no corner of the world will be dark enough” to hide Osama bin Laden, prime suspect in the Sept. 11 suicide attacks by hijacked airliners that killed about 5,600 people.

Mullah Omar, supreme leader of Afghanistan’s ruling Taliban movement that is protecting bin Laden, asked the world’s Muslims to help his impoverished country resist the U.S.-led raids.

PLEA FOR HELP

“Muslims should dissociate from it. Every Muslim having a strong faith should resolutely act against the egoistic power [America],” he told the BBC, monitored in Pakistan.

“They should extend any help and support they can to the Islamic Emirate of Afghanistan,” he said.

In the United States, a third person tested positive for exposure to anthrax in Florida, intensifying fears that the bacteria spores were deliberately spread.

“We have an additional employee that has been found to have anthrax present,” said FBI special agent in charge Hector Pesquera.

One man who worked at the offices of American Media Inc. in Boca Raton, Fla., has died. A mailroom employee was also exposed to the disease. The third person to test positive was a 35-year-old woman who also worked at the company, officials said.

Acting U.S. Attorney Guy Lewis said the investigation was now considered a criminal probe, but stressed there was no indication the anthrax was produced or caused by individuals related to the Sept. 11 attacks on New York and Washington.

U.S. security agencies are on high alert all over the world. Another U.S. official, on condition of anonymity, told Reuters that since Sept. 11 authorities had foiled bombing plots aimed at four U.S. embassies.

The official declined to identify the locations of the embassies or the plotters.