(Adds U.S. statement on special forces in Kut)

By Paul Tait

BAGHDAD, March 11 (Reuters) - At least 46 people were killed in a surge in violence across Iraq on Tuesday, including a roadside bomb attack on a bus carrying mourners and day-long clashes between gunmen and U.S. and Iraqi security forces.

Violence had fallen across Iraq by 60 percent since last June, but Tuesday's attacks underlined how fragile the security gains are.

In the southern city of Kut, members of anti-U.S. Shi'ite cleric Moqtada al-Sadr's Mehdi Army fought U.S. special forces and Iraqi security forces backed by U.S. warplanes in clashes in which 14 people died, security officials said.

Police at the general hospital in Nassiriya, 375 km (235 miles) south of Baghdad, said the casualties from the roadside bomb attack on a bus included women and children. Survivors said the bomb appeared to target a passing U.S. military convoy.

Police said the bus was carrying members of a family returning from mourning rites for a dead relative in the holy Shi'ite city of Najaf when it was hit about 60 km (40 miles) south of Nassiriya.

"There was blood and human flesh in the bus and on the floor. Shoes of men, women and children were everywhere," bus driver Zaji Abdul Hussein told Reuters.

Rahman Shaker, 60, covered in blood after carrying his badly wounded wife from the wreckage, said a U.S. convoy had just passed on the other side of the road when the bomb went off.

"I saw my wife covered in blood and took her out of the bus," Shaker said. "There were bodies covered in bloody blankets, and people screaming."

CLASHES

In Kut, 170 km (105 miles) southeast of Baghdad, Mehdi Army gunmen fought Iraqi security forces and U.S. special forces despite Sadr having renewed a six-month ceasefire last month. The cleric issued a statement at the weekend, however, saying they could defend themselves if attacked.

The commander of a quick reaction force in Kut, Lieutenant- Colonel Majid al-Amara, said the fighting was triggered by an attempt to arrest a Mehdi Army leader. He said 14 people were killed, including four gunmen, three children and a policemen.

The U.S. military gave a different account in a statement late on Tuesday. It said special forces had come to the aid of an Iraqi security patrol and come under attack by a large number of "suspected criminal militia fighters".

"The U.S. SF (special forces) returned fire, killing several enemy fighter and destroying two vehicles carrying machineguns," it said, adding a warplane had also destroyed a van suspected of transporting weapons and explosives.

Much of the fighting was reported to have died down by nightfall although sporadic shooting could still be heard.

Mehdi Army fighters have chafed at the extension of the ceasefire, complaining that it leaves them open to attack by U.S. forces and rival Shi'ite factions. The U.S. military says it only targets militiamen who are ignoring the ceasefire.

In separate clashes north of the capital, police said four Iraqi policemen, four gunmen and a civilian were killed in an attack on a security checkpoint in Mosul, which the U.S. military says is al Qaeda's last major urban stronghold in Iraq.

In Dhuluiya, also north of Baghdad, a suicide car bomber killed five people, including three members of a neighbourhood security unit, and wounded 14 in an attack on a checkpoint, police Lieutenant-Colonel Ibrahim al-Jubouri said.

U.S. military spokesman Rear Admiral Greg Smith acknowledged at the weekend there had been an increase in violence in Iraq but said the military did not believe it represented a trend. (Editing by Janet Lawrence)

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