Suspicious packages found in at least two locations abroad that were bound for the United States “apparently contain explosive material,” President Barack Obama said Friday, calling the discovery “a credible threat against our country.”

The packages led to increased searches of cargo planes and trucks in several U.S. cities, said law enforcement sources with detailed knowledge of the investigation.

U.S. officials believe that al Qaeda in the Arabian Peninsula, commonly referred to as AQAP, is behind the incident.

Obama confirmed that the packages originated in Yemen — the stronghold of al Qaeda in the Arabian Peninsula.

“We also know that al Qaeda in the Arabian Peninsula continues to plan attacks against our homeland, our citizens, and our friends and allies,” he said during a press briefing on the incident.

One suspicious package, found in the United Kingdom, contained a “manipulated” toner cartridge and had white powder on it as well as wires and a circuit board, a law enforcement source said. A similar package was discovered in Dubai, in the United Arab Emirates, the source said.

Both packages were bound for the United States, “specifically two places of Jewish worship in Chicago,” Obama said.

“Initial examination of those packages has determined they do apparently contain explosive material,” he said.

Authorities were looking for about 13 other packages shipped from Yemen, a law enforcement source said. Some of them had been found and an investigation of those had not indicated they are a threat, the source said.

There is no specific intelligence indicating the other packages are a threat or that they are in the United States, the source said, but authorities want to check them as a precaution.

A Yemeni diplomat in Washington said his government has opened a full-scale investigation into the incident but it was too early to speculate or reach any conclusions.

Investigators were looking for a “possible nexus to terrorism,” a U.S. official said.

“We are taking this very seriously,” the official said.

The plot could be a dry run to test Western security, another official told CNN.

The Department of Homeland Security said it “had taken a number of steps to enhance security,” including “heightened cargo screening and additional security at airports.”

“Passengers should continue to expect an unpredictable mix of security layers that include explosives trace detection, advanced imaging technology, canine teams and pat downs, among others,” DHS said in a statement. “As always, we remind the public to remain vigilant and report suspicious activity to local law enforcement.”

Some Jewish religious leaders in Chicago were alerted Friday, said Linda Haase, spokeswoman for the Jewish Federation of Metropolitan Chicago.

“We were notified about this earlier this morning,” she said. “We are taking appropriate precautions and we are advising local synagogues to do the same.”

Lucille Price, a receptionist at Anshe Emet Synagogue, said Chicago police made them aware of the reports and asked them to keep an eye out for suspicious packages among any deliveries that arrived Friday.

But congregation leaders at two prominent Chicago synagogues, Temple Sholom and Chicago Sinai Congregation, told CNN they were not made aware of any attempts to ship bombs or hazardous material to them.

In the United Kingdom, police were investigating the suspicious package at a freight distribution center at East Midlands Airport, about 100 miles north of London, said airport spokesman Russell Craig. U.K. Home Secretary Theresa May said the package, which originated in Yemen and was addressed to a U.S. destination, was discovered during a search of a cargo flight. The package is being examined, she said.

May sought to assure Britons that “safety and security of the U.K.” is her top priority.

“We are urgently considering what steps need to be put in place regarding security of freight originating from Yemen,” she said. “For security reasons there are currently no direct flights from Yemen to the U.K.”

She added that “at this stage there is nothing to suggest that any location in the U.K. was being targeted.”

Meanwhile, U.S. authorities seemed most focused on inspecting cargo planes.

Investigators examined two UPS planes that landed at Philadelphia International Airport and another at Newark Liberty International Airport in New Jersey, said Mike Mangeot, a UPS spokesman. Authorities later gave the “all-clear” at the airport in Newark, U.S. and U.K. officials said.

Authorities are focusing on flights coming from Yemen into the United States, according to the source.

In Philadelphia, six packages from Yemen were found aboard the two UPS planes that were sitting on the tarmac, said a law

enforcement source with knowledge of the investigation. Though there was no specific threat related to the two planes, U.S. authorities said they decided to check the cargo to be extra cautious, the source said.

All cargo on the planes was to be inspected, even packages that did not originate in Yemen, a process that will take several hours, the source said.

Hazardous material teams were using mobile equipment to check for biological, radioactive and chemical material as well as explosives, the source said.

One plane was parked in a remote area of the airport, by Gate 11. The other was near the UPS terminal, which is far from the passenger terminal.

The Transportation and Security Administration said authorities acted “out of an abundance of caution.”

UPS said it is cooperating with authorities, and its shipment is being removed from the aircraft.

In Newark, investigators examined another UPS plane, Mangeot said. Police determined that there was no threat.

In New York, the bomb squad responded to a report of a suspected explosive device inside a package aboard a UPS truck, said deputy police commissioner Paul Browne. Police later concluded that the truck at the Metro Tech Center facility contained nothing harmful.

-CNN