|Searches were conducted after several aircraft were grounded in Philadelphia, Newark and New York City
Authorities on three continents are investigating whether suspicious packages shipped from Yemen to religious sites in the US were part of a "terrorist plot".
No explosives have been found so far, but officials said on Friday they are investigating whether the packages were sent as part of a dry run for an attack.
Yemen is home to the al-Qaeda branch that tried to bomb a US-bound airliner on December 25, 2009.
Intelligence and law-enforcement officials discovered suspicious packages in Britain and Dubai, in the United Arab Emirates, late on Thursday night, prompting national security officials to alert Barack Obama, the US president, to a "potential terrorist threat", the White House said.
The two packages were addressed to Chicago religious sites, Ross Rise, a Chicago FBI spokesman, said.
One was a synagogue, said a US official who spoke on condition of anonymity to discuss the investigation.
The package in Britain, discovered aboard a jet in the East Midlands about two hours north of London, contained a toner cartridge with attached wires and powder.
It was found during routine screening of cargo, prompting authorities to search three aircraft and a lorry in the US on Friday, US officials said.
"Obama directed US intelligence and law-enforcement agencies, and the department of homeland security, to take steps to ensure the safety and security of the American people, and to determine whether these threats are a part of any additional terrorist plotting," the White House said in a statement.
Al Jazeera's Hashem Ahelbarra, reporting from Aden, said that the Yemeni authorities have denied claims that a direct cargo route from Yemen to the US exists.
"Authorities here continue to reiterate, however, that they are doing all they can to eliminate al-Qaeda from the country, amid growing international pressure," he said.
Yemeni officials said they launched a terrorism investigation and Scotland Yard said its investigators were testing a number of items seized from the jet in East Midlands.
In the US, searches were conducted in Philadelphia, Newark, New Jersey and New York City.
The packages were being sent via the shipping companies United Parcel Service (UPS) and FedEx. The packages, not the flights, originated in Yemen.
Kristin Lee, a Transportation Security Administration spokeswoman, said the jets in Philadelphia and Newark were "swept", and moved away from terminal buildings while law-enforcement officials investigated.
Two Philadelphia aircraft belonging to the Atlanta-based UPS were searched. A US law-enforcement official said nothing suspicious was found.
A source with knowledge of the situation in Newark who was not authorised to speak publicly said the FBI and a bomb squad checked two packages there and gave the "all clear".
Ray Kelly, the New York Police commissioner, said that the NYPD removed a package from a UPS lorry in Brooklyn, tested it for possible explosives and found it not to be dangerous.
The package was an envelope that came from Yemen, appeared to contain bank receipts, and was addressed to the JP Morgan Chase bank in Brooklyn, Kelly said.
The package arrived on a flight that landed at Kennedy airport, he said.
An Emirates flight from Dubai carrying another package from Yemen landed safely at Kennedy airport and was met by law-enforcement authorities. The aircraft was escorted by US fighter jets, a law-enforcement official said.
Mike Mangeot, a UPS spokesman, said two jets in Philadelphia that had come from Cologne, Germany, and Paris were being investigated.
"Out of an abundance of caution, those aircraft have been isolated, and they are looking into the shipments in question there," he said.
A third jet had arrived in Newark, New Jersey, from East Midlands airport. That flight was cleared and flew to UPS's main hub in Louisville, Kentucky, on its usual route, Mangeot said.
In central England, police evacuated a freight distribution building at East Midlands airport after a suspicious package was reported.
Police and emergency workers examined the package and lifted the security cordon by midmorning, but Leicestershire Constabulary later said officers were re-examining it "as a precaution".