One of two powerful bombs mailed from Yemen to Chicago-area synagogues travelled on two passenger planes within the Middle East, a Qatar Airways spokesman said today.

The US said the plot bears the hallmarks of al Qaida's offshoot in Yemen and vowed to destroy the group.

The airline spokesman said a package containing explosives hidden in a printer cartridge arrived in Qatar Airways' hub in Doha, Qatar on one of the carrier's flights from the Yemeni capital Sana'a.

It was then shipped on a separate Qatar Airways plane to Dubai in the United Arab Emirates (UAE), where it was discovered by authorities. A second, similar package turned up in at East Midlands airport on Friday.

He did not give any timeframe for the two flights in question - the airline operates daily passenger flights from Yemen that could also carry courier packages.

The plot was the latest to expose persistent security gaps in international air travel and cargo shipping nearly a decade after the September 11, 2001, attacks and showed terrorists appear to be probing those vulnerabilities.

In Washington, President Barack Obama's deputy national security adviser John Brennan said authorities "have to presume" there might be more potential mail bombs like the ones pulled from planes in the UK and the UAE.

US inspectors were heading to Yemen to monitor cargo security practices and pinpoint holes in the system.

An internal government report, obtained by The Associated Press, said the team of six inspectors from the Transportation Security Administration will give Yemeni officials recommendations and training to improve cargo security.

The report also says the agency is considering extending its security directive to increase inspection of cargo for all flights through November 8.

"We're trying to get a better handle on what else may be out there," Mr Brennan told NBC's Meet the Press as he made the rounds of the Sunday talk shows representing the Obama administration in the wake of the latest terrorist scare. "We're trying to understand better what we may be facing."

He told CNN's State of the Union programme that "it would be very imprudent... to presume that there are no others (packages) out there."

Brennan noted that because of the continuing threat, the world's largest package delivery companies - FedEx and UPS - have suspended air freight from Yemen.

Police in Yemen have arrested a young woman on suspicion of mailing the bombs and also detained her mother.

While he said "we feel good" about the steps taken since the thwarted plot, "I think we have to presume there might be" additional bombs.

The explosives, addressed to Chicago-area synagogues, were pulled off airplanes at East Midlands airport in Leicestershire and the UAE on Friday after intelligence officials were tipped off about them. That touched off a tense search for other devices.

After a six-hour sweep of cargo at the East Midlands airport, Leicestershire police came up empty and removed the security perimeter they had set up, British aviation safety consultant Chris Yates said.

But when officials in Dubai said they had discovered a bomb disguised as a computer printer cartridge, authorities urged the British to look again, a US official said.

"As a direct consequence, they put the cordon back up again and looked again and found the explosives," said Yates, relying on a report given to him by an eyewitness to the searches.

Mr Brennan called it "a very sophisticated device, in terms of how it was constructed, how it was concealed" and said it was a viable device.

"They were self-contained. They were able to be detonated at a time of the terrorists' choosing," Mr Brennan said, adding that officials are trying to determine whether the planes or the synagogues were the intended targets.

Al Qaida's offshoot in Yemen is suspected of mailing the bombs. The group was behind a failed bombing on a Detroit-bound airliner last Christmas that bore some of the same hallmarks as this plot.

In Yemen today, police were searching for additional suspects after arresting a female computer engineering student suspected of mailing the packages and also detaining her mother. Both arrests were on Saturday.

Yemeni President Ali Abdullah Saleh told reporters that the US and UAE had provided intelligence that helped identify the woman suspected of mailing the packages.

The 22-year-old Hanan al-Samawi is a student at the University of Sana'a, said Yemeni rights activist Abdel-Rahman Barman, of The National Organisation for Defending Rights and Freedoms. Her 45-year-old mother was arrested with her.

According to her university colleagues, al-Samawi is not known to be involved in any political activity or to have ties to any Islamic groups, Mr Barman said. He said she had not been allowed access to a lawyer.

Yemeni officials pointed to additional suspects believed to have used forged documents and ID cards. One member of Yemen's anti-terrorism unit said the other suspects had been tied to al Qaida.

US officials said suspects in the plot include the bombmaker suspected of designing the explosive used in the failed Christmas airliner bombing. The bombmaker is a key operative in al-Qaida's offshoot in Yemen, Al Qaida in the Arabian Peninsula.

"They are a dangerous group," Brennan said of al Qaida in Yemen. "They are a determined group. They are still at war with us and we are very much at war with them. They are going to try to identify vulnerabilities that might exist in the system," he said.

He said the US "will destroy that organisation as we are going to destroy the rest of al Qaida."