Washington/London, Oct 30 (DPA) President Barack Obama warned of a "credible terrorist threat" to the US Friday after two packages containing explosive materials were found on US-bound cargo flights at airports in Britain and Dubai.

The packages had originated in Yemen, the hub of Al Qaeda in the Arabian Penninsula, and both were addressed to Jewish synagogues in Chicago.

First indications were that two devices "apparently contained explosive materials", Obama said, though officials would not say whether the packages could have been activated as a bomb.

Obama's top counterterrorism advisor, John Brennan, said the packages were "in a form that was designed to carry out some kind of attack".

Brennan spoke over telephone with Yemeni President Ali Abdullah Saleh, who pledged his full cooperation in the investigation.

The discovery prompted authorities in Britain and the US to monitor a series of other flights coming from Yemen. Planes at airports in Philadelphia, New York and Newark, New Jersey, were all isolated and inspected for potential threats.

"The events of the past 24 hours underscore the necessity of remaining vigilant against terrorism," Obama said. He had ordered authorities "to take whatever steps are necessary to protect our citizens from this type of attack".

The first package was discovered early Friday on a UPS cargo plane during a stopover at East Midlands airport, near Nottingham in central England. The device was described by police as a "manipulated toner cartridge".

British Home Secretary Theresa May said the government was "urgently" reviewing steps to improve the security of cargo flights coming from Yemen, though there was never a specific threat to Britain itself.

A second suspicious package was discovered on a flight operated by FedEx and intercepted in Dubai. Authorities in the United Arab Emirates said the device was brought to a laboratory for investigation.

Both FedEx and UPS said they were halting all flights out of Yemen and were cooperating with authorities.

Obama said that Al Qaeda in the Arabian Peninsula "continues to plan attacks" and that the US was working with Yemeni authorities to "destroy this Al Qaeda affiliate".

Brennan said intelligence received Thursday night first led investigators to inspect the cargo flights. Obama was immediately alerted to the threat.

With security tightened, two other UPS cargo planes were isolated and checked for suspicious items at a US airport in Philadelphia and one plane was checked in Newark, New Jersey.

Another flight from the UAE to New York's JFK airport was escorted by fighter jets to its destination because it contained cargo from Yemen, authorities said. There were also reports of a UPS truck being stopped in the New York City borough of Queens.

US officials said there had been no specific bomb threats at US airports, but that security sweeps were being taken "out of an abundance of caution". Security and cargo screening was also stepped up at other airports, the Department of Homeland Security said.

The cordon around the cargo distribution centre at East Midlands airport had also since been lifted and "police have gone home", police in nearby Leicester said.

The two suspicious packages were addressed to Jewish synagogues in Chicago, Brennan said. The US Anti-Defamation League in a statement said it had alerted Jewish communal institutions across the US to increase their mailroom security as a result of the threat.