WASHINGTON - US law enforcement agents were conducting a manhunt for at least three suspects linked to a credible but unconfirmed terrorism threat ahead of the 10th anniversary of the September 11, 2001 attacks, ABC News said on Thursday.

More details of "credible" terror threat emerged Thursday night of individuals trying to carry out attacks on the United States using truck or car bombs, while President Barack Obama has ordered increased security.

Quoting intelligence officials, the report said at least three individuals -- one believed to be a US citizen -- entered the country in August in an attempt to launch a "vehicle-borne attack against Washington DC or New York around the anniversary of 9/11."

The suspected terrorists were believed to be coming from Afghanistan,  and traveled through at least one other country, the report said.

It also said US law enforcement agencies are seeking at least two rental trucks nationwide. Those trucks were missing from locations in Kansas City, Missouri. It was not immediately known whether the search was connected to the plot.

White House officials said President Obama was briefed on the matter, and he had ordered federal agencies to boost security.

Word of the threat came earlier in the evening, and authorities were investigating a "specific, credible threat" -- the first received in relation to the 9/11 anniversary.

Meanwhile, NBC News reported a "specific, credible threat" -- the first received in relation to the 9/11 anniversary, quoting anonymous US sources as saying.

The threat, which would target New York City or Washington, DC, "has more credibility attached to it than the rest of the increasing chatter picked up in the past few days," though not imminent and not confirmed, the report said.

Officials are considering issuing a statement on the threat after the president's jobs speech before a joint session of Congress Thursday night, according to the report.

The US is ramping up security measures around the country as the 10th anniversary of the 9/11 attacks approaches. A variety of commemorative events will be held in New York City, the Pentagon in Northern Virginia and Shanksville, Pennsylvania, where terrorists hijacked passenger planes and crashed them on September 11, 2001, killing nearly 3,000 people.

The White House said Wednesday that the administration is taking "all necessary steps" to guard against possible terror plots coinciding with the anniversary, as terrorists have in the past shown interests in anniversaries.

Earlier this month, Secretary of Homeland Security Janet Napolitano said the US remains at "a heightened state of vigilance" and security measures are in place to detect and prevent plots against the United States should they emerge.