President Obama said Friday that he directed national security officials to take "whatever steps are necessary" to protect the country after a "credible terrorist threat" was discovered.
Speaking in the White House briefing room, Obama said that suspicious packages on cargo planes originating in Yemen "do apparently contain explosive material."
"The events of the past 24 hours underscore the necessity of remaining vigilant against terrorism," Obama said.
John Brennan, Obama's counterterrorism czar, said that the terror threat level is not being raised and there was no discussion of the president authorizing the shooting down of any planes.
Obama was notified of the suspicious packages on Thursday night. British officials discovered a toner cartridge that was rigged with wires and white powder, which tested negative for explosives. Other packages originating from the same Yemen address were tracked down and searched.
"The American people should know that counterterrorism professionals are taking this threat seriously," Obama said.
White House press secretary Robert Gibbs said after the president spoke that Obama instructed all relevant national security agencies to "take steps to ensure the safety and security of the American people and to determine whether these threats were a part of any additional terrorist plotting."
An official told The Associated Press that U.S. authorities are probing whether the incident was a dry run for a bomb plot, but John Brennan, Obama's counterterrorism czar, said he does not think dry run would have included using explosive materials.
Gibbs said in a statement that Obama was notified of the potential attacks at 10:35 p.m. Thursday by Brennan, the president's top adviser for homeland security and counterterrorism.
"Based on close cooperation among U.S. government agencies and with our foreign allies and partners, authorities were able to identify and examine two suspicious packages, one in East Midlands, United Kingdom, and one in Dubai," Gibbs said. "Both of these packages originated from Yemen. As a result of security precautions triggered by this threat, the additional measures were taken regarding the flights at Newark Liberty and Philadelphia International airports."
Gibbs said the president has received regular updates on the situation, and Obama has no plans to change his campaign schedule either Friday night or over the weekend.
Brennan and Gibbs answered questions about the threat after Obama spoke at the late-afternoon press conference.
Two suspicious packages were addressed to religious institutions in Chicago, according to the FBI, putting all churches and synagogues on alert in the city Friday.
Yemen has come under pressure from both the White House and Congress to rein in extremist elements in the country. The al Qaeda branch that claimed responsibility for the failed bombing attempt of an airliner over Detroit on Christmas Day — al Qaeda in the Arabian Peninsula — is headquartered in Yemen.
The incident also comes on the heels of the State Department issuing a travel alert for Americans going to Europe because of al Qaeda threats against targets there.
This story was updated from an earlier version