BAGHDAD, Iraq -- A standoff over a multimillion-dollar security bill owed by the Iraqi government shut down Baghdad's international airport yesterday 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 2 1/2 years after the U.S.-led invasion. Many Iraqis and some foreign contractors -- vital to rebuilding Iraq -- blamed the transitional government for yesterday'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.

Yesterday, Global's security contractors maintained their posts around the airport but turned back would-be travelers -- shutting down travel without actually leaving unguarded either the airport road, which was one of Iraq's most-bombed routes until U.S. military greatly intensified its presence there, and the airport, 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 traveler 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 Transportation Ministry officials, when called at midmorning yesterday for comment on the closing, said they had not known about it.

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 U.S. forces to step in.

Acting Transportation Minister Esmat Amer vowed to send Iraqi troops to force reopening of the airport. The ministry dispatched its police, only to call them back after they reached the U.S. checkpoint. "We did not want to create a confrontation," Amer said. Interior Ministry officials 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 today, however, it was unclear when that would happen.

The shutdown was more than an inconvenience. Insurgent attacks, banditry and the numerous armed men of murky affiliation on Iraq's roads make driving out of the country gravely dangerous for Iraqis and almost impossible for foreigners.

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. Yesterday, 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.

And a new U.N. human-rights report 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."