Security concerns forced the U.S. consulate in Karachi to close Tuesday. Roads around the building have been closed and additional police are standing guard.

The consulate sent employees home and shut its doors Tuesday morning after receiving word of a possible security threat.

"For security reasons we don't discuss the specific nature of the threat. We've received credible information of a threat that caused heightened security concerns," said Greg Crouch, the U.S. Embassy spokesman in Pakistan.

A Pakistani police official reportedly received a telephone call threatening an attack, which prompted the consulate's closure.

The embassy also issued a warning to all Americans in region, advising them to avoid the consulate and the nearby Marriott Hotel.

In June 2002, a car bomb outside the consulate killed 12 Pakistanis and injured nearly 50 others.

Last year a bomb was discovered near the consulate but was safely defused.

Karachi police chief Tariq Jamil says officers have already responded to the latest threat.

"We are there and we are deployed temporarily," he said. "We have closed the road, we are checking the vehicles."

U.S. officials say they will reopen the consulate as soon as security permits, probably by Wednesday.

Karachi is Pakistan's largest city and commercial center. It is also a key base for Pakistani militants who opposed the U.S. invasion of Afghanistan in 2001 and are angry at the Pakistan government's support for the U.S.-led war on terrorism.