Yemen has launched a massive manhunt for two militants accused of involvement in the failed parcel bomb plots on US-bound flights.
The aim of the military operation is to capture suspected bombmaker Ibrahim al-Asiri, and the US-born radical preacher Anwar al-Awlaki, wanted by Washington for his links to al-Qaeda.
Yemeni authorities also began the trial in absentia of Mr Awlaki.
Meanwhile, militants blew up an oil pipeline in central Yemen.
Yemeni officials said the blast in southern Yemen's Shabwa province carried the "fingerprints of al-Qaeda".
US officials suspect al-Qaeda in the Arabian Peninsula (AQAP) was behind the parcel bomb plot.
Early on Tuesday, Yemen put Mr Awlaki on trial in absentia, accusing him and two other men of plotting to kill foreigners and being members of AQAP.
It was the first legal action by Yemen against Mr Awlaki, and came as the country faces heavy pressure to crack down on the militant network following the interception of two mail bombs intercepted in Dubai and Britain last week.
Mr Awlaki and suspected Saudi bombmaker Ibrahim Hassan al-Asiri are key suspects in the failed parcel bomb plot.
"Asiri is believed to be hiding and moving with senior al-Qaeda elements such as Nasser al-Wuhayshi. Security intelligence are still tracking them down to exactly identify their whereabouts," a security official told the Reuters news agency.
Security forces had been deployed to parts of Marib and Shabwa governorates, where the two men are believed to be hiding under the protection of local tribes.
Officials in Britain have said that the crucial tip-off that led to the discovery of the bomb came from Jabr al-Faifi, a repentant AQAP member who handed himself in to authorities in Saudi Arabia two weeks ago.
One bomb travelled on two passenger planes before being seized in Dubai.
The other almost slipped through Britain, and UK authorities have been criticised for their initial failure to find the bomb on a plane at East Midlands airport.
Amid efforts to track down the militants behind the parcel bombs, officials were also searching for the attackers on the pipeline in Shabwa province.
The militants had placed a bomb under the pipeline in a town east of the provincial capital, Ataq, reports said.
South Korea's Yonhap news agency said that state-owned Korea National Oil Corporation operated the 204km (128 mile) pipeline.
Armed groups have repeatedly attacked Yemen's oil pipelines in the past, some of them members of tribes disgruntled with the authorities in the impoverished Arabian peninsula country.