In this story:

• Other missions closed due to threats
• Authorities: When, not if, bin Laden will strike
• Intensified efforts to crush network

WASHINGTON (CNN) -- U.S. law enforcement and intelligence officials are engaged in an intense, but largely secret global campaign to counter a series of credible new threats from terrorist leader Osama bin Laden and his far-flung network of sympathizers.

Government officials Monday extended the closure of the U.S. embassy in Uganda after receiving a new bomb threat in Kampala, sources told CNN.

The threat was received by the U.S. ambassador on her cell phone. Officials are concerned about how the caller was able to get the private number.

One U.S. official said that while "the caller did not state any (specific) affiliation," the threat is believed to be an "...Islamic threat, possibly (linked to) bin Laden."

Great Britain, whose embassy is in the same complex as the U.S. facility in Kampala, also decided to temporarily close its embassy in response to the threat received Sunday.

Other missions closed due to threats

Similar threats considered to be credible forced the closure last week of the U.S. outposts in Cameroon and Botswana.

Law enforcement sources told CNN the new threats are part of a trend seen since the United States bombed several of bin Laden's camps last summer. Since then, the number of serious threats against U.S. embassies has more than doubled, the sources said.

Authorities are particularly concerned about U.S. embassies in poor countries where security is weaker, especially countries where extremists affiliated with bin Laden are known to reside.

As a result, any remotely credible threat in such a country is taken seriously and may result in the closing of an American facility. In December, the State Department shut down a number of African embassies after receiving a credible threat in Ethiopia.

Authorities: When, not if, bin Laden will strike

"We're just waiting to see where bin Laden tries to strike next," said one official monitoring the global chess game.

Authorities have been saying for weeks: It's not a question of "if bin Laden will strike next, but when."

Although bin Laden is believed to be linked to some past terrorist acts in Yemen, FBI agents who returned to Washington from Yemen on Monday believe bin Laden operatives were not involved in last month's deadly kidnapping of Western tourists.

Intensified efforts to crush network

The U.S. government campaign to crush bin Laden's ability to strike has led to an aggressive effort to locate, intimidate and extract information from individuals with possible ties to the Saudi expatriate's network.

"They're really putting a squeeze on these people," said a counterterrorism expert in Washington.

One individual who may fit that category is an Albanian man released from custody Monday. The Albanian security guard was detained last week after he was suspected of watching U.S. diplomats in Tirana, possibly to report to bin Laden sympathizers, officials said.

Bin Laden has been charged in New York with masterminding the August 7, 1998, truck bombings of two U.S. embassies in east Africa which killed 12 U.S. citizens and more than 200 Kenyans and Tanzanians.

Of the 12 persons indicted in New York on charges related to the bombings, bin Laden and seven others remain at large, and four are in federal custody.

Correspondent Pierre Thomas, Justice Department Producer Terry Frieden and National Security Producer Chris Plante contributed to this report.