(Adds Maliki quote)

By Wisam Mohammed

BAGHDAD, March 24 (Reuters) - Iraqi Prime Minister Nuri al-Maliki reviewed security operations on Monday in the southern oil hub of Basra, where authorities imposed a night-time curfew after clashes between police and Mehdi Army militia.

Rival factions of Iraq's Shi'ite majority and criminal gangs are competing for control of Basra, Iraq's second city and gateway to the Gulf, along with its oil fields which are the source of most government revenues.

"Basra is subjected to unjust attacks and many parties from inside and outside are taking part in this. The attacks target the security and stability of the city," said Maliki, who travelled with his top security ministers.

"The federal government because of its commitment to support the regional government in Basra decided to re-impose security, stability and law in this city."

Two powerful Shi'ite factions, the Supreme Islamic Iraqi Council and the followers of Moqtada al-Sadr, are fighting for power in Basra with a smaller Shi'ite party, Fadhila.

Criminal gangs are also vying for control of lucrative oil-smuggling routes at a time when Iraq, which has the world's third-largest reserves of oil, is trying to boost exports.

The power struggle has been marked by assassinations and kidnappings. The city's police chief has escaped a number of attempts on his life.

CURFEW IMPOSED AFTER CLASHES

After clashes in Basra between police and members of Sadr's Mehdi Army militia, authorities imposed a night-time curfew on Monday on the movement of people and vehicles in the province.

"To impose the law and chase the criminals ... we have decided to impose a curfew in the whole of Basra starting from tonight at 10 p.m. until 6 a.m. and every day after until further notice," said Lieutenant-General Mohan al-Furaiji.

Authorities said they would also temporarily close schools and universities this week as well as access to Basra, 550 km (340 miles) southeast of Baghdad, from neighbouring provinces.

The Mehdi Army has kept a low profile since Sadr called a ceasefire last August and extended it last month.

But gunbattles in Baghdad and the southern city of Kut last week have raised fears that the ceasefire may be unravelling at a time when the U.S. military is withdrawing 20,000 troops.

Iraqi security forces took control of Basra from British forces in December, although 4,100 British troops remain at an airbase outside the city to offer assistance if needed. Plans to reduce that force by mid-year appear to have been delayed.

"The Iraqi government will not ask for the British forces' help and they will not have any role in Basra," Iraqi government spokesman Ali al-Dabbagh said. (Additional reporting by Randy Fabi; Writing by Ross Colvin; Editing by Dominic Evans)

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