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 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.

In the United States, 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, although the investigation was continuing on at least two.

Obama’s sobering assessment 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 that Arab country 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.

“The use of commercial cargo carriers demonstrates the continuing determination of terrorists to exploit any vulnerabilities in our security systems,” said U.S. Sen. Susan Collins, R-Maine.

“The positive test of two of these suspicious packages for explosives confirms the danger posed by this most recent attempted terrorist attack.”

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.

Intelligence personnel had been monitoring a suspected plot for days, officials said. The packages in England and Dubai were discovered after Saudi Arabian intelligence picked up information related to Yemen and passed it on to the United States, 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.

In the hours after the discoveries, Yemeni officials and Scotland Yard were investigating and the United States issued a 72-hour ban on all cargo from Yemen.

U.S. authorities conducted searches of aircraft in Philadelphia, Newark, N.J., and New York City.

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.