WASHINGTON Calling it a "credible terrorist threat," President Barack Obama said apparent explosive material was found on two U.S.-bound packages from Yemen, triggering searches of flights with other packages from Yemen and an investigation into whether al-Qaida was behind a new terror plot.

Sources told NBC News that both packages contained toner cartridges with wires and white powder. The devices were found in Britain and Dubai last night.

"An initial examination of those packages has determined that they do apparently contain explosive material," Obama told reporters.

The devices were "in a form that was designed to try to carry out some kind of attack," added White House counterterrorism adviser John Brennan. "Clearly, from the initial observation, the initial analysis that was done, the material that was found ... was intended to do harm."

The two suspicious packages "have been made inert," he added.

Homeland Security said in a statement it was taking new measures, "including heightened cargo screening and additional security at airports."

A law enforcement official told NBC that the two packages were addressed to a synagogue and a Jewish community center in Chicago.

One U.S. official said authorities are investigating whether the incident was a dry run for a plot to send bombs through the mail delivery system.

Yemen is the home of al-Qaida in the Arabian Peninsula, the offshoot branch that claimed responsibility for an attempted bombing of a U.S.-bound airliner last Christmas.

One device was found during a stopover in Britain. A UPS cargo flight had been bound for Chicago but was at a British airport when the cartridge was spotted.

Officials found the suspicious item during basic security screening.

In Chicago, synagogues were warned to be on alert Friday.

"We were notified this morning that synagogues should be on the alert," Linda Haase, associate vice president of the Jewish Federation of Metropolitan Chicago, told Reuters. "We are taking appropriate precautions and are advising local synagogues to do likewise."

TSA issues alert
The Transportation Security Administration earlier said that cargo flights that landed safely at Newark and Philadelphia airports were being searched after "reports of potentially suspicious items onboard."

Two jets in Philadelphia belonging to UPS were searched. A federal law enforcement official told the AP that nothing suspicious was found.

The flight that landed at Newark, N.J., also was a UPS cargo jet. After the jet was searched, officials gave the all clear.

In New York, an Emirates commercial flight arrived from Dubai around 3:30 p.m. ET and was also being searched as a precaution.

The flight is carrying one of some 15 packages from Yemen that the U.S. wants to inspect, WNBC said.

"This is only because there is cargo from Yemen on the flight," said FBI spokesman Richard Kolko. "There is no known threat associated with this cargo or this flight."

Earlier Friday, a UPS truck was searched and then cleared in Brooklyn.

Al-Qaida active in Yemen
The United States has stepped up its training, intelligence and military aid to Yemen after the failed Christmas Day plot, for which the Yemeni wing of al-Qaida claimed responsibility.

The accused Christmas Day bomber, Umar Farouk Abdulmutallab, has told U.S. investigators he received the explosive device and training from al-Qaida militants in Yemen.

Yemen has been trying to quell a resurgent branch of al-Qaida, which has stepped up attacks on Western and government targets in the Arabian Peninsula country.

One official said intelligence personnel had been monitoring a suspected plot for days. The packages in England and Dubai were discovered late Thursday after a foreign intelligence service picked up information related to Yemen and passed it on to the U.S., this official said.

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