A policeman stands guard in front of the Capitol Hill in Washington, capital of the United States, on Sept. 9, 2011. U.S. Homeland Security Secretary Janet Napolitano on Friday asked Americans to be vigilant against potential terror threat surrounding the 10th anniversary of the 9/11 terrorist attacks. [Xinhua/Zhang Jun]

U.S. Homeland Security Secretary Janet Napolitano on Friday asked Americans to be vigilant against potential terror threat surrounding the 10th anniversary of the 9/ 11 terrorist attacks.

"As we head into the 9/11 anniversary weekend, we continue to urge the American public to be vigilant and report any suspicious activity to law enforcement authorities," Napolitano said in a statement.

She promised to take all threat reporting seriously, including the recent "specific, credible but unconfirmed" threat information disclosed by U.S. officials on Thursday night.

At least three individuals -- one believed to be a U.S. citizen -- entered the country in August with the intent to launch a vehicle-borne attack against Washington D.C. or New York around the 9/11 anniversary, officials said.

"We continue to be in close contact with our federal, state and local law enforcement partners to ensure that all steps necessary to mitigate any threats are taken," Napolitano said.

White House press secretary Jay Carney said President Barack Obama was briefed Friday morning by his national security team, including the president's chief anti-terrorism advisor John Brennan, National Security Advisor Tom Donilon and his deputy Denis McDonough, on the threat.

The president urged the whole team to "redouble its efforts to follow all the information we get, all the threat information we get, and to take all the necessary precautions," Carney told reporters aboard Air Force One as Obama flew to a political event in Virginia.