The US has been sending unarmed drones over Mexico since February to gather intelligence on major drug cartels, the Mexican government has confirmed.

墨西哥政府证实，自二月开始，美国多次向其境内派遣无人侦察机收集主要贩毒集团的情报。

The high-altitude drones are not visible from the ground
The flights were made at Mexico's request and were supervised by the Mexican air force and other agencies.

It is the latest sign of growing US involvement in Mexico's campaign against violent drugs gangs.

Mexico confirmed the missions had been taking place after they were revealed by the New York Times newspaper.

Most of the drone flights have been over northern border areas, the scene of much of the drug-related violence that has left more than 34,000 dead since late 2006.

The New York Times report said the missions had been kept secret because of Mexican legal restraints（限制，禁止） and sensitivities over sovereignty（主权，独立） .

In a statement, Mexico's National Security Council said the high-altitude flights had been carried out with "unrestricted respect" for Mexican law.

"These operations are always carried out with the authorisation, vigilance and operative supervision¹ of national agencies, including the Mexican Air Force," the statement said.

Valuable intelligence

It added that the targets of surveillance were determined² by Mexican authorities, and that information gathered had been "particularly useful".

Unnamed US officials told the New York Times that drones had gathered intelligence that led to the arrest in Mexico of several suspects in connection with the murder of a US immigration agent, Jaime Zapata.

US President Barack Obama and his Mexican counterpart, Felipe Calderon, formally agreed to continue the surveillance flights during talks in Washington on 3 March, which included a frank exchange of grievances³（抱怨，不平） , Mexican and US officials said.

In state department cables released by Wikileaks and published by The Guardian⁴ newspaper last December, the US ambassador to Mexico, Carlos Pascual, painted an unflattering portrait of the Mexican security forces, and questioned whether President Calderon could win his war on drugs.

Foreign military and law enforcement agents can only operate in Mexico under extremely limited conditions, according to the Mexican constitution.

But the rising violence in Mexico has seen the US and Mexico deepen their co-operation to tackle a common threat.