WRAPUP 1-Yemen hunts parcel bombers, al Qaeda suspected

* Parcels found in Britain, Dubai set off global alert

* Saudi Arabia tipped off United States

* Yemen forces search for plotters

* Dubai police say bomb bears al Qaeda hallmarks

By Mohamed Sudam and Mahmoud Habboush

SANAA/DUBAI, Oct 30 (Reuters) - Yemeni forces searched on Saturday for suspected al Qaeda militants behind a plot to bomb Jewish targets in Chicago, uncovered by the interception in Britain and Dubai of parcels with explosives sent from Yemen.

U.S. President Barack Obama said on Friday U.S. authorities would spare no effort to find the source of the packages, which he called a "credible terrorist threat" aimed at two places of Jewish worship.

One parcel intercepted in Dubai contained a bomb hidden in a printer and bore all the hallmarks of al Qaeda, Dubai police said on Saturday.

The parcel contained explosive pentaerythritol trinitrate (PETN) in a printer and cartridge, police said. PETN was the material used in a failed plot to bomb an airline over the U.S. in December 2009.

"The parcel was prepared in a professional way where a closed electrical circuit was connected to a mobile phone SIM card hidden inside the printer," the statement said.

"This tactic carries the hallmarks of methods used previously by terrorist organisations such as al Qaeda."

The bomb also contained lead azide, which is used in detonators. Dubai police experts defused the device, the statement said.

SAUDI HELP

The White House said Saudi Arabia helped identify the threat from Yemen, which has become a haven for some anti-American militants, while Britain and the United Arab Emirates also provided information.

Yemeni security forces set up checkpoints across Sanaa on Saturday, searching vehicles and carrying out identity checks.

Dozens of heavily armed police and military forces were scattered across the Yemeni capital, including the diplomatic quarter and the large ring road around the city, stopping cars and questioning passengers, a Reuters witness said.

Yemen had also stepped up security at its air and seaports, a security official told Reuters.

Obama said security would be increased for American air travel for as long as necessary. U.S. officials said they were searching for more packages that could have come from Yemen.

Obama's counterterrorism adviser, John Brennan, told reporters: "We were onto this. We were looking for packages that were of concern."

Of the plotters, Brennan said: "Clearly they are looking to identify vulnerabilities in our system. We've been able to stay ahead of them."

The security threat unsettled Americans just days before they vote in midterm congressional elections that have been dominated by economic woes rather than the issue of terrorism.

AQAP SUSPECTED

Suspicion fell on al Qaeda in the Arabian Peninsula (AQAP), whose militants operate out of Yemen and claimed responsibility for a failed plot to blow up a U.S. plane over Detroit on Christmas Day in 2009.

The group is affiliated with al Qaeda, whose militants killed about 3,000 people using hijacked passenger jets in the Sept. 11 attacks on the United States in 2001.

"Initial examinations of those packages has determined that they do apparently contain explosive material," Obama said at a press briefing at the White House.

PETN was the same chemical explosive used in the bomb sewn into the underwear of a Nigerian man who has been charged with attempting to blow up an airliner over Detroit last Christmas, a plot hatched in Yemen.

The White House said "both of these packages originated from Yemen" and Obama was informed of the threat on Thursday.

One of the packages was found on a United Parcel Service cargo plane at East Midlands Airport, about 160 miles (260 km) north of London. The other was discovered at a FedEx Corp facility in Dubai.

UPS and FedEx, the world's largest cargo airline, said they were halting shipments from Yemen. UPS planes were searched and then cleared in New Jersey and Philadelphia.

One U.S. official and some analysts speculated that the parcels may have been a test of cargo screening procedures and the reaction of security officials.

"This may be a trial run," the U.S. official said.

(Reporting by Michelle Nichols in New York, Alister Bull and Jeff Mason in Washington and Mohammed Abbas in London; Raissa Kasolowsky, Mohamed Sudam, Mahmoud Habboush, Amran Abocar in Sanaa and Dubai, writing by Peter Millership)