FACTBOX: The Strait of Hormuz, Iran and the risk to oil

(Reuters) - A cargo ship contracted by the U.S. military fired on an Iranian vessel in Gulf waters on Friday, raising tension between Washington and Tehran in a region vital to world oil shipments.

In January, the United States said Iranian boats aggressively approached three U.S. navy battle ships, warning them they would explode in minutes. Iran has said it was a routine contact.

Analysts say Iran, OPEC's second largest crude producer, could seek to impede traffic through the strategic waterway if the United States resorts to military action against the Islamic Republic over its disputed nuclear program.

U.S. naval chiefs are concerned Iran could resort to mining the strait and the wider Gulf in a major conflict.

The sea channel which shares Iran's coastline at the entrance to the Gulf is the world's most important waterway because of the huge volume of oil exported through it daily.

-- Oil flows through the Strait account for roughly 40 percent of all globally traded oil supply, according to the U.S. Energy Information Administration (EIA). The figure fluctuates with changing OPEC output.

-- In May 2007, the International Energy Agency (IEA) estimated 13.4 million barrels per day (bpd) of crude passing through the narrow channel on tankers, down from 16-17 million bpd before a round of OPEC production cuts.

-- An additional 2 million barrels of oil products, including fuel oil, are exported through the passage daily as well as liquefied natural gas.

-- Exports from the world's largest liquefied natural gas exporter Qatar also pass through the Strait en route to Asia and Europe, totaling some 31 million metric tons a year.