BAGHDAD, March 2 (Reuters) - The U.S. military said on Sunday it did not know when it would clear al Qaeda fighters from the northern Iraqi city of Mosul, the Sunni Islamist group's last urban stronghold in Iraq.

Tens of thousands of U.S. and Iraqi forces are carrying out offensives against al Qaeda in central and northern Iraq. Iraqi Prime Minister Nuri al-Maliki promised in January there would be a "decisive push" against its fighters in Mosul.

"Mosul is the centre of al Qaeda terrorist activities today," U.S. military spokesman Rear Admiral Greg Smith told a news conference.

"I suspect we will make all sufficient efforts towards achieving our goals as rapidly as possible and the security of Mosul is certainly one of those goals. But the precise time in the future that will occur will be difficult to predict."

Smith said the ethnically and religiously mixed city, 390 km (240 miles) north of Baghdad, was al Qaeda's strategic hub, providing access to foreign fighters as well as funding.

"Between half and two-thirds of attacks we chart across Iraq each day occur in and around this city," Smith said, saying coalition forces had killed or captured 142 al Qaeda fighters in Mosul since the start of January.

On Wednesday, he said an air strike had killed an al Qaeda suspect named as Abu Yassar al-Saudi, who was accused of masterminding an attack in eastern Mosul in January that killed five U.S. soldiers.

Paulos Faraj Rahho, the Chaldean Catholic archbishop of Mosul, was seized on Friday after gunmen attacked his car in the city's eastern al-Nour district. Police have given no details of who is thought to be behind the abduction.

Chaldeans belong to a branch of the Roman Catholic Church that practises an ancient Eastern rite and form the biggest Christian community in Iraq.

The Syrian Catholic archbishop of Mosul, Basile Georges Casmoussa, who was also kidnapped at gunpoint in 2005 but freed a day later, is among those now negotiating for Rahho's release, according to media reports.

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