MEXICO CITY — Mexico's raging drug war claimed the lives of six more police officers, ambushed on patrol in the marijuana-rich state of Sinaloa, authorities said Friday.

The ambush follows the assassination Thursday of a senior police commander, part of a long string of killings apparently aimed at eroding public confidence in the government's ability to challenge drug gangs.

The six officers were killed when two carloads of heavily armed men cut off their patrol vehicle in the Sinaloa capital of Culiacán, an official with the state Attorney General's Office said by e-mail.

More than 4,400 people have been killed in drug violence in Mexico, among them hundreds of police, since President Felipe Calderón launched an all-out offensive against drug cartels after taking office in December of 2006.

Calderón insists the surge in killings and gun battles is a sign of his government's success in hitting drug-trafficking networks. But several analysts suggest the high-profile killings, in particular, make the government and its main law enforcement agencies appear vulnerable.

The assassinations, along with the gangs' growing propensity to decapitate victims and make threats brazenly via posters and the Internet, "have a clear objective," Interior Minister Juan Camilo Mouriño said Friday, "to intimidate, frighten, paralyze society, and with that, force the federal government to retreat."

Inspector Igor Labastida, a senior officer in the Federal Police, was the fifth top commander killed in the last 13 months. A gunman armed with an Uzi submachine gun shot Labastida and one of his bodyguards as they ate lunch at a small, busy restaurant in Mexico City on Thursday. The gunman escaped in a waiting car while a second man videotaped the bloodied bodies and calmly walked away, witnesses told the Mexican newspaper El Universal.

Two bodyguards were wounded in the attack.

Labastida survived an earlier assassination attempt, and his name figured on a hit list purportedly drawn up and circulated by drug gangs. Another senior commander on the list, Edgar Millán Gómez, was assassinated in May.

Meanwhile, the Mexican government Friday applauded U.S. Senate approval of a $400 million aid package for Mexico's drug war that will provide the Calderón government with training, telecommunications, aircraft and other equipment.

Mexico earlier objected to portions of the bill, known as the Mérida Initiative, that would have required it to change the way human-rights violations are investigated. But congressional officials agreed to soften those conditions.

Mouriño, the interior minister, praised the measure because it represented "a concrete expression of the principle of shared responsibility" in the drug war.

Mexico has long complained that it endures the ravages of the war while the U.S. has done little to stop the flow of guns southward into the hands of the cartels. Mouriño said he thought that was changing and that U.S. authorities had begun to track and stop weapons more efficiently.

"Are we totally satisfied with what is being done? Not yet," he said during a news conference Friday. "But we are satisfied at having made the U.S. government aware of the level of the problem, what it represents for our country and the need to take steps on the U.S. side."