US authorities were investigating a potential threat, of uncertain credibility, in connection with the inauguration of president-elect Barack Obama, the US Homeland Security Department said today.

The threat concerned an unspecified incident involving the Somali Islamist insurgent group al Shabaab, authorities said.

Obama was due to be sworn in as president just before noon EST (1700 GMT) and huge crowds gathered in central Washington for the event. Security was unprecedented, with tens of thousands of US troops and police.

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"The FBI, the Department of Homeland Security ... and the intelligence community are coordinating with other law enforcement authorities to investigate and analyze recently received information about a potential threat on inauguration day," department spokesman Russ Knocke said.

"This information is of limited specificity and uncertain credibility," he said.

The FBI and Homeland Security Department notified law enforcement around the United States of the potential threat on Monday, authorities said, speaking on condition of anonymity.

The bulletin was part of regular efforts to keep local officials aware of security developments. There was no change to the overall threat level, the department said.

A law enforcement official said the United States had been tracking the al Shabaab-related threat for several days.

The Somali group is on the US list of terrorist groups. It has primarily acted in the violence-wracked Muslim East African country, but US intelligence officials have recently expressed concern about young Somalis living in the United States who have gone to train with al Shabaab.

Obama's transition team had been briefed on the threat warning, Knocke said.

He said the public should continue with their plans to attend the inaugural, but be vigilant.

"Inauguration events could present an attractive target due to the large public gatherings and participation of many dignitaries," Knocke said. "Authorities are constantly reviewing security measures in light of this threat information."