WASHINGTON — Authorities on three continents thwarted multiple terrorist attacks aimed at the United States from Yemen on Friday, seizing two explosive packages addressed to Chicago-area synagogues and packed aboard cargo jets.

The plot triggered worldwide fears that al-Qaida was launching a major new terror campaign. President Barack Obama called the coordinated attacks a "credible terrorist threat," and U.S. officials said they were increasingly confident that al-Qaida’s Yemen branch, the group responsible for the failed Detroit airliner bombing last Christmas, was responsible.

Parts of the plot might remain undetected, Obama’s counterterror chief warned. "The United States is not assuming that the attacks were disrupted and is remaining vigilant," John Brennan said at the White House. One of the packages was found aboard a cargo plane in Dubai, the other in England.

Preliminary tests indicated the packages contained the powerful industrial explosive PETN, the same chemical used in the Christmas attack, U.S. officials said. The tests had not been confirmed. In the U.S., cargo planes were searched up and down the Eastern Seaboard, and an Emirates Airlines passenger jet was escorted down the coast to New York by American fighter jets.

No explosives were found aboard those planes, though the investigation was continuing on at least two. Obama’s sobering assessment, delivered from the White House podium, unfolded four days before national elections in which discussion of terrorism has played almost no role. The president went ahead with weekend campaign appearances.

The terrorist efforts "underscore the necessity of remaining vigilant against terrorism," the president said. While he said both packages that contained explosives originated in Yemen, he did not explicitly assign blame to al-Qaida, which is active in the Arab nation and long has made clear its goal of launching new attacks on the United States. Authorities in Dubai intercepted one explosive device.

The second package was aboard a plane searched in East Midlands, north of London, and officials said it contained a printer toner cartridge with wires and powder. Brennan said the devices were in packages about the size of a breadbox. While Obama didn’t specifically accuse Yemen’s al-Qaida branch, Brennan called it the most active al-Qaida franchise and said anyone associated with the group was a subject of concern.

The radical U.S.-born Muslim cleric Anwar al-Awlaki, who now is in hiding in Yemen, is believed to have helped inspire recent attacks including the Fort Hood shooting, the Times Square bombing attempt and the failed Detroit airliner bombing last Christmas Day. Another American hiding in Yemen, Samir Khan, has declared himself a traitor and has helped produce al-Qaida propaganda. Most of the officials spoke on condition of anonymity, citing the ongoing investigation.

Brennan later told reporters that the explosives "were in a form that was designed to try to carry out some type of attack," but he provided no further details. "The forensic analysis is under way," he said, adding, "Clearly from the initial observation, the initial analysis that was done, the materials that were found in the device that was uncovered was intended to do harm." Intelligence personnel had been monitoring a suspected plot for days, officials said.

The packages in England and Dubai were discovered after a Saudi Arabian intelligence picked up information related to Yemen and passed it on to the U.S., one official said. U.S. intelligence officials warned last month that terrorists hoped to mail chemical and biological materials as part of an attack on America and other Western countries using the mail.

The alert came in a Sept. 23 bulletin from the Homeland Security Department and obtained by The Associated Press. In the hours following the discoveries, Yemeni officials and Scotland Yard were investigating and the U.S. issued a 72-hour ban on all cargo from Yemen.

"As a precaution, DHS has taken a number of steps to enhance security," the agency said in a statement. "Some of these security measures will be visible while others will not." U.S. authorities conducted searches of aircraft in Philadelphia, Newark, N.J., and New York City.

Since the failed Christmas bombing of a Detroit-bound airliner, Yemen has been a focus for U.S. counterterrorism officials. Before that attack, the U.S. regarded al-Qaida’s branch in Yemen as primarily a threat in the region, not to the United States. The Yemen branch known as al-Qaida in the Arabian Peninsula has since become a leading source of terrorist propaganda and recruiting. Authorities believe about 300 al-Qaida members or cells operate in Yemen.

The Yemeni government has stepped up counterterrorism operations, with help from the U.S. military and intelligence officials. Mohammed Shayba, general-director of the state airline’s cargo department, said the government is conducting an investigation. "Those in charge are in constant meetings and they are investigating and taking the issue seriously," he told The Associated Press.