Popular unrest sweeping the Middle East highlights the US military's reliance on Arab regimes that offer privileged access to airfields and ports from Cairo to Qatar.

The military's dominant role in the region hinges on a web of agreements with friendly Arab states that allow American forces to patrol oil shipping routes in the Gulf, target Islamist militants and keep a watchful eye on arch-foe Iran.

Roughly 27,000 US forces are deployed at an array of bases and sites throughout the Gulf, along with a 50,000-strong contingent in Iraq and thousands more aboard naval ships, a US military official told AFP.

Major air fields in Qatar and the United Arab Emirates, a large base in Kuwait and the US Navy's Fifth Fleet headquarters in Bahrain serve as key points in an arc around Iran, ensuring American forces can move swiftly with heavy firepower.

In Bahrain, where security forces have cracked down on street protests after popular revolts ousted leaders in Tunisia and Egypt, about 4,000 Americans are stationed as part of the US Navy's Fifth Fleet headquarters.

With a flotilla of aircraft carriers, destroyers and amphibious ships at its disposal, the Fifth Fleet oversees an area spanning the Red Sea, the Persian Gulf and the Arabian Sea.

Losing the headquarters in Bahrain would be a setback but not a catastrophe, as the Navy could move the command post elsewhere, said David Aaron, a former diplomat and a senior fellow at the Rand Corporation think tank.

No single base or agreement represents an Achilles heel, but taken as a whole, the network is a crucial linchpin for American military power, he said.

"What could be at stake here is an ability to have forces in the Gulf to reassure our allies out there, our partners, that they will be protected from Iran," he said.

"If we lost all of those because of the turmoil that would be a blow to our ability to deter Iran."

Instead of Cold War-style bases housing large numbers of troops, the US military depends on overflight rights, the use of air bases in the western Gulf and special access to the strategic Suez Canal, where American naval ships receive top priority.

Losing swift passage through the canal or broad access to Egypt's air space "would either diminish our military mobility or sharply increase time and cost to overcome them," Aaron said.

With some 15 refueling depots and logistical centers spread through the Gulf, tanks, ammunition, fuel and supplies for US forces can be moved into position as needed, with contractors helping to manage the effort.

US commanders are always looking to expand their network of partners in the region while working to accommodate regimes that prefer American forces keep a low profile to avoid stirring up popular resentment.

In 2003, the US military shut an air base and scaled back its presence in Saudi Arabia -- amid acrimony surrounding the Iraq invasion -- and shifted air operations to Qatar and Oman.

Some bases are shrouded in secrecy, including Al Dhafra Air Base in the United Arab Emirates, which the Pentagon does not officially acknowledge. The Air Force reportedly uses the airfield to fly U2 spy plane missions and tanker aircraft for the wars in Iraq and Afghanistan.

Since the September 11 attacks, air bases in Djibouti, Oman and elsewhere reportedly have provided vital platforms for launching drone missile strikes against Al-Qaeda operatives in nearby countries.

Last year, the United States reportedly moved Predator unmanned aircraft to a base at a Yemeni Red Sea port.

In Yemen, where anti-regime protests have raged for days, up to 100 US troops help the country's battle against Al-Qaeda militants.