SULAYMANIYAH, Iraq, Jan 12 (Reuters) - Iraq's government has cut off petrol supplies to the breakaway Kurdish-run north of the country, Kurdish officials said, sending prices soaring and ordinary Kurds rushing to stock up on fuel.

The officials said they did not know why the fuel supplies, brought across the front lines between Iraqi government troops and the Kurdish north in tankers, car fuel tanks and gerry cans, had been shut off for a second day on Sunday.

But the move comes amid U.S. preparations for a possible war in Iraq over Baghdad's alleged weapons of mass destruction, with thousands of American troops being despatched to the Gulf.

The apparent embargo points up the fragility of the Kurds' de facto autonomy from Baghdad won when U.S. and British planes began enforcing a no-fly zone over the area in 1991 after Iraqi troops put down an uprising against President Saddam Hussein.

Aside from a small oilfield in the east of the area and a converted refinery near the city of Sulaymaniyah which once refined sugar, north Iraq's three million Kurds rely almost entirely on supplies brought from the government-held region.

Pump prices had more than quadrupled on Sunday compared with before the blockade, some petrol stations closed down altogether for lack of fuel while long queues formed at others as drivers sought to fill up while they could.

One Kurdish official in the city of Sulaymaniyah, in the east of the rugged enclave, said petrol had been cut off before due to wrangling over prices with Baghdad.

Iraqi Kurds would join other opposition groups in running the country under U.S. scenarios for a post-Saddam Iraq should a U.S.-led invasion topple the present government.

The Kurds played a leading role in mustering opposition parties in a conference in London last month and are due to host a further meeting of leaders opposed to Saddam near the eastern city of Arbil later this month.