Mexico admitted Wednesday that unarmed US drones have flown over its territory to gather intelligence on organized crime, showing increased, politically-sensitive cooperation in its drug war.

"The Mexican government has requested from the US government, on specific occasions and events, the support of unmanned planes to obtain specific information" on security, the National Security Council said in a statement.

It said the planes had flown mainly along the US-Mexico border, which stretches some 2,000 miles (3,000 kilometers), and that Mexican officials had set the objectives for the missions.

The flights complied with Mexican law, the statement said.

The New York Times earlier reported that the Pentagon began flying unmanned missions "deep into Mexican territory" last month -- fueling concerns about Mexico's ability to bring its drug gangs to heel under an extensive military crackdown launched by President Felipe Calderon in 2006.

The United States already provides millions of dollars in aid to train Mexican security forces, and cooperates with Mexico on intelligence.

The Times cited unnamed US officials as saying that the drones had gathered vital information leading to Mexico's arrest of several suspects in last month's killing of US immigration and customs agent Jaime Zapata, 32.

It said US President Barack Obama and Calderon formally agreed to the drone missions in a meeting on March 3 but kept it secret because of possible political and legal constraints.

Mexican politicians have often criticized the involvement of US agencies on Mexican soil, and even members of Calderon's conservative National Action Party (PAN) expressed concerns Wednesday.

"Our laws don't allow any (foreign) police or military force to operate inside our country," Felipe Gonzalez, a PAN politician and president of the Senate Security Commission, told AFP.

Mexico's security council, meanwhile, underlined the "enormous demand for drugs, arms supplies and availability of financial resources" which made the United States a key player in the drug war.

The relevations came amid bilateral tensions sparked by recent allegations that US agents allowed hundreds of guns to be smuggled into Mexico as part of drug trafficking investigations.

The Times also quoted Mexican and US officials as saying that Mexico has been turning a blind eye to US wiretapping of the telephones of drug suspects and to US agents carrying weapons in violation of longstanding Mexican restrictions.

"It wasn't that long ago when there was no way the DEA could conduct the kinds of activities they are doing now," Mike Vigil, a retired chief of international operations for the Drug Enforcement Administration, told the Times.

DEA Administrator Michele Leonhart declined to confirm the flights, speaking in Washington Wednesday, but she said: "We are always looking for new methods ... to collect information."

Leonhart added that due to an "unprecedented partnership" with the Mexican government, "we have had significant successes in the past several years."

Although Mexican and US officials have lauded the recent captures or killings of a string of high-level Mexican drug traffickers, they have had little visible impact on the ground.

More than 34,600 people have killed in rising Mexican drug violence since 2006, according to official figures.

The US military has used missile-firing drones to attack insurgents along the border of Afghanistan and Pakistan in recent years, angering many in Pakistan who see the attacks as a violation of their sovereignty.