A standoff over a multimillion-dollar security bill owed by the Iraqi government shut down Baghdad's international airport Friday and severed Iraq's last safe route to the outside world, highlighting disarray in the country's administration and security forces and spurring U.S. troops to step in to maintain security.

With the closing, air travel joined electricity, clean water and security as essential services now in short supply in Iraq 21/2 years after the U.S.-led invasion. Many Iraqis and some foreign contractors, who are vital to rebuilding Iraq, blamed the transitional government for Friday's shutdown.

The dispute concerned a payment, now totaling $36 million, owed British-based Global Strategies Group for running the airport's security. The $4.5 million monthly contract was signed by Iraq's previous government and has gone unpaid since January as the current government tries to renegotiate it, Iraqi officials confirmed. Global shut down airport operations for 48 hours in June in a dispute over the same contract.

On Friday, Global's security contractors turned back would-be passengers, shutting down travel. But they maintained their posts around the airport, guarding the airport road, which was one of Iraq's most frequently bombed routes until the U.S. military intensified its presence there, and the airport itself, which insurgents have not managed to hit.

"Make a U-turn. There are no flights today," a Global guard at a sandbagged, concrete-walled checkpoint told one traveler, a police officer with luggage in the back of the car and a ticket in hand for a training seminar in neighboring Jordan.

"Why?" the man asked, demanding to know when he could fly. "We don't know," the guard answered. "We just need you to turn around."

The news caught more than travelers by surprise; top Iraqi officials from the Transportation Ministry, called at midmorning for comment, said they were unaware of the closing.

By late afternoon, U.S. troops had set up their own impromptu checkpoint by parking Humvees across the airport road and stopping each vehicle to check for IDs. Lt. Col. Steve Boylan, a U.S. military spokesman, said the Iraqi government had asked the Americans to step in.

Acting Transportation Minister Esmat Amer vowed to send Iraqi troops to force a reopening of the airport. "This issue is related to Iraq's sovereignty. Nobody is authorized to close the airport," Amer told the Associated Press.

The ministry dispatched its police, only to call them back after they reached the American checkpoint. "We did not want to create a confrontation," Amer said. Officials from the Interior Ministry also briefly appeared at the checkpoint, guards said.

Government officials said throughout the day that the airport would reopen imminently and normal traffic would resume. By early Saturday, however, it was unclear when that would happen, although a spokesman for Global said the airport would reopen Saturday at 8 a.m.

The shutdown was more than an inconvenience. Insurgent attacks, bandits and the numerous armed men of murky affiliation on Iraq's roads made driving out of the country gravely dangerous for Iraqis and almost impossible for foreigners. Disappointed travelers, including parents with children returning or leaving home after summer breaks and a doctor who needed to send a sick 5-year-old to India for surgery, besieged travel agents.

"We are suffering from some of the inexperienced ministers," Fadhil Mahdi, a merchant desperate to get goods into the country, said at the home of his travel agent, where he had gone for help. "They must think of the people who will be affected by their wrong decisions."

The shutdown has the potential to create major headaches for companies doing business in Iraq, said Ron Cruse, president and chief executive of Logenix International, LLC, a Springfield, Va.-based logistics firm with contracts here.

Cruse also said he was concerned about the precedent set for dealings between Iraq ministries and foreign companies, at a time when the Iraqis are taking over the management of an increasing number of contracts. "Contractors are not looking forward to doing business with the ministries for exactly this reason," Cruse said.

Contractors, particularly security firms, play a major role in Iraq.

In the northeastern city of Tall Afar, meanwhile, U.S. forces kept up bombardment of a neighborhood that has become a stronghold for insurgents. Airstrikes by U.S. helicopter gunships and jets killed 18 suspected insurgents, the U.S. military said.

U.S. and Iraqi forces have massed 5,000 of their troops at Tall Afar for an expected ground assault on the district. On Friday, troops laid a 24-hour curfew on the city.

Residents and hospital officials in the western border city of Qaim said jets bombed a suspected safe house of Abu Musab Zarqawi's al Qaeda in Iraq movement. Three foreign fighters and five Iraqis were killed, said Faraj Kubaisi, a hospital physician. There was no confirmation from the U.S. military.

And a U.N. human rights report released Thursday said authorities were losing ground in their struggle to restore the rule of law in Iraq.

The report, prepared by the U.N. Assistance Mission for Iraq, cited "serious allegations of extra-judicial executions taking place which underline a deterioration in the situation of law and order."

Staff writer Griff Witte in Washington, correspondent Jonathan Finer in Tall Afar and special correspondents Bassam Sebti and Omar Fekeiki in Baghdad contributed to this report.

American soldiers refuel a tank at an Army base in Tall Afar, where U.S. forces bombarded an insurgent stronghold.