Mexico's foreign minister on Thursday defended the government's decision to allow unarmed US drones over Mexico, in a rowdy session before the Senate on a controversial development in the drug war.

"It's the exclusive power of the federal executive to grant permission for overflights of any nature," Patricia Espinosa told a plenary session.

Authorities could use "any method of collecting information" under a Mexican security law, Espinosa said.

The use of unarmed, intelligence-gathering drones would not be permanent, she added, without giving a time frame for the program, made public Wednesday by The New York Times.

Espinosa also underlined that foreign troops had not crossed into Mexico, a move that the constitution bars without permission from senators.

The National Security Council admitted Wednesday that unarmed US drones have flown over Mexico to gather intelligence on the country's powerful drug gangs.

It said that Mexican officials had set the objectives for the missions.

The Times reported that the Pentagon began flying 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.

"Never before has so much been surrendered for so little and with such mediocre results," said Rosario Green, a former foreign minister from the main opposition Institutional Revolutionary Party (PRI), on Thursday.

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

Recent allegations that US agents allowed hundreds of guns to be smuggled into Mexico as part of drug trafficking investigations have also raised tensions over bilateral cooperation.

Mexico is "dissatisfied" with US policies to stop weapons crossing its border, Alejandro Poire, the national security spokesman, said Thursday.

According to Mexico, more than 80 percent of weapons decomissioned from its drug gangs come from the United States.

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