By James Vicini

WASHINGTON (Reuters) - The United States still faces a high risk of another terror attack and should stay on heightened alert even though Osama bin Laden's al Qaeda network has been significantly disrupted by the U.S. military campaign in Afghanistan, FBI Director Robert Mueller said on Friday.

"We, as a country, number one, ought to stay on high alert and understand that there are still those out there with both the capability and willingness to attack us in a variety of ways," Mueller told a news briefing.

He said the U.S. military effort had been "exceptionally successful," arresting a number of al Qaeda leaders, disrupting its operations and taking away the sanctuary "where additional terrorists were trained day in and day out" and plotted terror attacks around the world.

Mueller was responding to comments by Senate Majority Leader Tom Daschle questioning the White House plans to expand the war on terrorism and saying bin Laden must be found for the war effort to succeed.

The FBI has been investigating the Sept. 11 hijacked plane attacks on the World Trade Center and the Pentagon, which killed about 3,000 people. The United States has blamed bin Laden and al Qaeda for the attacks.

In its most recent alert, the FBI warned of another possible terror attack in the United States or against U.S. interests in Yemen on or around Feb 12.

HIGH TERROR ATTACK RISK

Although that date has passed, Mueller said the United States still was at "a high level of risk for terrorist attacks" and should remain on a "very high state of alert."

He said the hijackers came into the United States in small groups, operating in communities around the country and avoiding contact with any al Qaeda supporters or sympathizers who may have been known to the FBI.

In other comments, Mueller praised Pakistan's cooperation in investigating the kidnapping and murder of U.S. reporter Daniel Pearl. FBI agents have been in Pakistan, assisting Pakistani authorities.

Asked whether the killers might be tied to al Qaeda, Mueller said, "It is fair to say the investigation is ongoing in Pakistan as to the links of those who are responsible for the kidnapping and killing of Daniel Pearl to other terrorist organizations."

Mueller also declined to rule out any connection between the Sept. 11 attacks and the mailing of anthrax-laced letters late last year which killed five people in the United States.

"We're still open to that possibility. I cannot say that there is no tie. We are looking at everything and until we have completed the investigation ... I could not definitely be able to say that there was no tie-in," he said.

FBI investigators, however, believe the anthrax most likely was sent by someone in the United States, probably a scientist.

Mueller dismissed claims by some scientists that the FBI has moved too slowly in the anthrax investigation. "I don't think in any way, shape or form we have been dragging our feet," he said.

Despite investigating since October, the FBI is not close to identifying a suspect in the attacks in which anthrax-laced letters were sent to two U.S. senators and to the news media last fall, FBI officials have said.