SOKOTO, Nigeria (Reuters) - Sunni Muslims attacked the headquarters of minority Shi'ites in Nigeria's far northwestern state of Sokoto for the second time in four days on Monday, wounding several people, a Shi'ite leader said.

The rival groups have waged bloody street battles in the Islamic city, on the fringes of the Sahara desert, for about two months. Residents say about a dozen people have been killed.

The state governor said the attacks were politically motivated and called on religious authorities to intervene after one person was killed and 14 wounded on Friday.

Heavily armed riot police cordoned off the area around the Shi'ite compound on Monday after it was attacked by Sunnis wielding bows and arrows and machetes, a witness told Reuters.

"I suspect that the police are protecting the Sunnis," said Shi'ite leader Kasimu Umar by telephone from the compound, adding that several people were injured in Monday's attack.

The dispute is ostensibly about doctrinal differences between the two groups and access to the city's central mosque.

But Sokoto State Governor Attahiru Bafarawa said the real cause was political rivalry between his opposition All Nigeria People's Party and the ruling People's Democratic Party (PDP).

"As far as I'm concerned, what is happening is political and because of that I wrote three petitions to President (Olusegun) Obasanjo complaining of the PDP role in instigating the problem," Bafarawa told a radio phone-in on Sunday.

Although Bafarawa is a Sunni, he has argued that Shi'ites should have access to the central mosque and is often heckled by members of the public who accuse him of siding with Shi'ites.

"I am only a governor, but issues of religion belong to the Sultanate Council, which is in charge of mosques and preaching, so I appeal to people not to allow themselves to be used by politicians in killing each other," he said on Sunday.

More than 20,000 people have been killed in religious, ethnic and communal violence in Nigeria since the oil-exporting country returned to democracy six years ago.

The nation's 140 million people are roughly equally divided between Muslims in the north and Christians in the south.