ADEN, (Reuters) – Yemeni police clashed with suspected al Qaeda militants in Aden on Friday, arresting several, as they searched for a group that had attacked an intelligence building there, a security official said.

The Interior Ministry put all its security forces on alert in the southern port city to prevent militants from entering or smuggling in arms to destabilise the economic hub of the impoverished Arabian Peninsula state.

The security official said clashes broke out before dawn during a house-to-house search in Aden’s Saada area, launched in an attempt to arrest militants behind the attack on the city’s intelligence headquarters that killed 11 people last Saturday.

“Some suspects were arrested,” the official told Reuters, declining to give more details.

New clashes erupted in mid-day, residents said.

The opposition Sahwa Net website said nine people were arrested overnight, one of whom had died in detention. Police officials could not be reached to comment.

Yemen has blamed al Qaeda’s wing in Yemen for the attack in which militants wearing military uniforms raided the intelligence police building, killing seven security officers, three women and a 7-year-old boy, and freeing several detainees.

Authorities said earlier they arrested the head of the group behind the assault.

Yemen, a neighbour of top oil exporter Saudi Arabia, has been a growing security concern for the West since the Yemeni-based arm of al Qaeda claimed responsibility for an unsuccessful attempt to set off a bomb on board a U.S.-bound airliner in December.

The Interior Ministry “called on security bodies to tighten their grip on Aden’s coast and to keep it under constant watch to prevent the infiltration of any terrorist elements into the city or the smuggling of weapons”, the ministry website said.

The ministry “reiterated the importance of safeguarding Aden province, the economic capital of Yemen, from any terrorist act”, the website added.

Yemen is struggling to curb a separatist movement in the south and cement a ceasefire with Shi’ite rebels in the north. It is under international pressure to quell domestic conflicts to focus on a growing al Qaeda presence in the country.

In the capital Sanaa, unidentified gunmen shot and wounded Abdulraqib al-Qirshi, an opposition leader who had returned home earlier this month after more than 30 in exile in Syria, a senior member of his Arab nationalist party told Reuters.

A day before Saturday’s attack, al Qaeda’s Yemen-based regional branch threatened to respond to a state crackdown against it in eastern Yemen, calling on local tribesmen to take up arms against the government.