Washington, Aug 4 (PTI): The US has increased security at all its major airports and railway stations, and has taken a number of measures, that include temporary closure of its 22 diplomatic missions, following a "very specific" al-Qaida threat emanating from the Arabian Peninsula.

President Barack Obama himself is keeping a close tab on the developments and the threat, which the officials did not disclose.

National Security Adviser Susan Rice held a meeting of top US national security officers to review the potential terror threat coming from the Arabian Peninsula.

The President, who early this week, had instructed his national security team to take all appropriate steps to protect the American people in light of the threats, himself is receiving briefings regularly on this issue and keeping a close tab on the developments.

"Given nature of the potential threat, throughout the week, assistant to the President for homeland security and counter-terrorism Lisa Monaco has held regular meetings with relevant members of the inter-agency to ensure the US Government is taking those appropriate steps," the White House said.

However, it did not give details of the terrorist threat and where it is emanating from.

Rice on Saturday chaired a meeting with the Principals Committee to review the situation and follow-up actions.

Obama has received frequent briefings over the last week on all aspects of the threat reports and the preparedness measures. He was again briefed by Rice on the developments.

On Friday, the State Department issued a worldwide travel alert and close down 22 embassies and consulates on Sunday in view of the al-Qaida threat.

Congressman Peter King, chairman of the Sub-committee on Counter-terrorism and Intelligence, said the threat was very specific, but he did not divulge any details.

"We're not certain exactly where something might happen but it's very specific as to when and it's also very specific as to the fact that it is going to happen, so we have to be on alert everywhere," King told a news channel.

In view of the terrorist threat, the Department of Homeland Security had yesterday increased security measures at airports, train stations and other transportation hubs. It expanded scrutiny of visitors coming into the US.