Iraq's oil police have stepped up patrols to protect installations against a possible surge in al Qaeda attacks as U.S. troops withdraw, the head of the force said on Tuesday.

Multibillion-dollar deals Baghdad signed with energy majors could quadruple oil output capacity to Saudi levels within six years but that depends on the OPEC member securing oilfields, refineries and other vital infrastructure.

Major General Hamid Ibrahim, head of Iraq's energy protection force, said half of all attacks planned by al Qaeda targeted the country's oil sector. His force has so far managed to foil most attempts, he said.

There is direct targeting of the oil sector ... By the start of the withdrawal there will be attacks not just on oil, but they (insurgents) will try to rattle the situation in the country, he told Reuters in an interview. We are ready and on alert.

But a bombing attack late on Tuesday on an oil pipeline in the oil hub of Basra raised questions over the ability of the oil police to halt attacks. Three bombs hit a pipeline that transports crude from southern oilfields to storage tanks, setting the pipeline on fire.

Although Iraq took responsibility for the security of its oil sector in 2005, the United States has still been providing aerial surveillance and other support to battle Sunni insurgents and Shi'ite militia, who have plagued the country since the U.S.-led invasion in 2003.

But by the end of December - nearly nine years after the U.S.-led invasion - only a small contingent of civilian trainers and fewer than 200 U.S. military personnel will remain.

HUMMER DELIVERY

The Iraq-Turkey pipeline in the north, which carries around a quarter of Iraq's oil exports, is regularly hit by sabotage, usually blamed on al Qaeda and former members of Saddam Hussein's banned Baath party.

And in early June, militants blew up a storage tank at the Zubair 1 storage facility near Basra, despite tight security.

Ibrahim said Iraqi security forces had foiled more than four plots against the nearby southern Doura refinery and were now coordinating with the Iraqi air force to monitor oil sites and pipelines.

The poorly equipped force has also received Hummer military vehicles and other supplies from U.S. forces as they pack up, he said.

We used to dream of having a few cars to reinforce our forces, now we have thousands, he said. Now we have good equipment, guns and bullets. It is a positive thing.

The government has built blast walls and watch towers and installed cameras and is talking to foreign investors such as British major BP to train the force, he said.

But Ibrahim added that his 40,000-strong force was still stretched, especially in the vast west of the country.

We have shortages and we can't say we are self-sufficient... The worry that we have now is that some oilfields in the western parts are vast fields, he said.

U.S. officials say that Iraq's oil security forces are up to the task but coincide they need to improve further.

(Editing by Patrick Markey and Ben Harding)