(Refiling to add dropped word in second paragraph)
* U.S. drones searching Mexico for drug gangs
* Mexico approved program; says sovereignty not issue
* Former foreign minister attacks decision (Recasts with Mexican reaction)
By Patrick Rucker
MEXICO CITY, March 16 (Reuters) - Mexico said on Wednesday it was allowing U.S. spy planes to enter its airspace to track drug smugglers, prompting opposition cries that the government had sold out to Washington.
The Mexican government said use of the unmanned drones was part of stepped-up joint efforts to fight the traffickers, whose conflict with the state has cost more than 36,000 lives over the past four years.
Authorities divulged details of the flights following a report in The New York Times on Wednesday that said the aircraft had uncovered information useful to officials investigating the killing of a U.S. customs agent.
Opposition leaders reacted angrily to the news, with former foreign minister Rosario Green calling it outrageous that Mexican national sovereignty had been compromised without the consultation of Congress.
The controversy is a fresh headache for President Felipe Calderon, who has staked his reputation on bringing the drug cartels to heel.
The drug war's constant bloodletting has left a stain on his administration, and Calderon's conservative National Action Party is now well behind the main opposition party in opinion polls leading up to next year's presidential election.
Green, a senator from the opposition Institutional Revolutionary Party, told daily Reforma that use of the drones was a "clear violation of Mexican sovereignty" and that the decision was made without consulting lawmakers.
"What else does president Calderon need to do to hand over control of the country? It's very worrying," she said.
Calderon's national security spokesman Alejandro Poire said bringing in drones had given Mexican authorities an edge over the cartels, and that they were being used mainly on the U.S-Mexican border.
"This isn't a general surveillance. They're being used for specific, concrete missions," he told Mexican Radio Formula.
U.S authorities responded with outrage when Immigration and Customs Enforcement agent Jaime Zapata was shot on a highway north of Mexico City on Feb. 15 in an attack by suspected drug gang members that also wounded his partner. [ID:nN23527360]
The Times said the Pentagon began flying the drones over Mexico last month.
The U.S. Department of Homeland Security (DHS) said only that it was working with Mexican authorities on a wide variety of law enforcement missions.
"All U.S. cooperation with Mexico is at the government of Mexico's invitation and is fully coordinated with the government of Mexico," said DHS spokesman Matthew Chandler, declining comment on the investigation into Zapata's killing.
This explanation was not good enough for Ricardo Monreal, a senator for the opposition labor party.
"This simply pushes the country into a state of indignation, rendering it subordinate and too submissive to the neighboring country to the north," Monreal said. (Additional reporting by Dave Graham in Mexico City and Jeremy Pelofsky in Washington; Editing by Xavier Briand)
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