"With the success of previous raids, we believe we are now able to carry out strikes more or less around the clock as we wish,'' Defense Secretary Donald H. Rumsfeld said at the Pentagon.

Administration officials declined to specify the targets of the renewed attacks. But anti-aircraft fire could be heard in the Afghan capital, Kabul, and Taliban officials reported bombs falling around their headquarters in Kandahar and the northwestern city of Herat.

Four weeks after the worst terrorist attack on American soil, President Bush moved quickly to beef up his new Office of Homeland Security and was conferring with German Chancellor Gerhard Schroeder, whose nation has offered to assist in military operations.

In Florida, the FBI continued its investigation into the death of one man from anthrax, and the exposure of his co-worker to the deadly disease.

"It remains a situation of concern with the federal government,'' said White House spokesman Ari Fleischer, as officials offered antibiotics as a precaution to hundreds who worked in the same building as the two men. Fleischer added, however, that "it's not unusual at times like this for false alarms to go off.''

At the Pentagon, Rumsfeld said the United States had no independent verification of the reported deaths of the U.N. workers, or whether they might have died after being hit by anti-aircraft fire.

"Nevertheless we regret the loss of life,'' he said.

"If there were an easy way to root terrorist networks out of countries that harbor them, it would be a blessing, but there is not,'' he added.

The bombing marked the third straight night of attacks by American-led forces. But even before night fell, pilots flew strikes sporadically throughout the day, a sign of increased confidence that whatever air defenses the ruling Taliban regime possessed had largely been suppressed.

Taliban officials said Osama bin Laden, the suspected mastermind behind the Sept. 11 terrorist attacks in New York and Washington, had survived the American assault.

Abdul Salam Zaeff, the Taliban envoy to Pakistan, told reporters the United States had spurned Afghanistan's request for evidence of bin Laden's involvement. "America is sending warplanes, bombs and cruise missiles in place of evidence,'' he said.

Administration officials said there was proof enough - even before bin Laden's videotaped message released on Sunday praising the attacks that killed thousands and taunting America as a nation afraid.

One day after former Pennsylvania Gov. Tom Ridge was sworn in as the nation's first director of homeland security, the administration was adding new top-level officials to the effort. Administration sources said Richard Clarke, who heads the government's counterterrorism team, will direct the security effort for the nation's information systems. Retired Gen. Wayne Downing was tapped to work with military and intelligence resources, these officials said.

U.N. officials confirmed the deaths of four security guards for a mine-clearing program in Afghanistan, the first independent verification of civilian casualties in the U.S.-led war effort.

U.N. spokeswoman Stephanie Bunker said the four had been spending the night in their office, on the edge of Kabul, and in close proximity to a radio transmission target that was a target of bombs.

"It was assumed they were safe where they were,'' said Bunker, speaking from the Pakistani capital of Islamabad. "Otherwise they would have been relocated for sure.''

In an appeal to the United States, she said, "People need to distinguish between combatants and those innocent civilians who do not bear arms.''

"It's inevitable there will be mistakes that take place in a situation where the lines remain unclear,'' said Sen. John McCain, R-Ariz. McCain, who made his comments in an interview on ABC's "Good Morning America,'' was a Navy pilot during the Vietnam war, and an American prisoner of war.

Pilots flying from the USS Enterprise in the Arabian Sea returned from Monday night's bombing runs with unused live bombs, the carrier's captain said.

But Rumsfeld said: "We're not running out of targets, Afghanistan is. And I would add, that they are emerging as we continue.''

American officials have identified bin Laden as the leading force behind the Sept. 11 attacks that killed more than 5,000 people at the World Trade Center, the Pentagon and in a plane crash in the southwestern Pennsylvania countryside.

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