Other administration officials said a US Homeland Security drone helped Mexican authorities find several suspects linked to the killing of a US Immigration and Customs Enforcement agent, Jaime Zapata, on February 15.

The US President, Barack Obama, and his Mexican counterpart, Felipe Calderon, agreed to continue the surveillance flights during a White House meeting on March 3. The American assistance has been kept secret because of legal restrictions in Mexico and the heated political sensitivities there about sovereignty, the officials said.

Before the outbreak of drug violence in Mexico that has left more than 34,000 dead over the past four years, such an agreement would have been all but unthinkable, they said.

US and Mexican officials declined to comment publicly about the introduction of drones to combat the drug trade. But some officials, speaking only on the condition of anonymity, said the move was evidence of the deepening co-operation between the two countries in their efforts to prevail over a common threat.

In addition to expanding the use of drones, the two leaders agreed to open a counter-narcotics ''fusion'' centre, the second such facility in Mexico, where Mexican and American agencies would work together, the officials said.

Before Mr Obama met Mr Calderon at the White House, diplomatic tensions threatened to weaken the co-operation between their governments.

US State Department cables obtained by WikiLeaks had reported criticism of the Mexican government by US diplomats, setting off a storm of resentment in Mexico. Then in February, outrage in Washington over Mr Zapata's murder prompted Mexican officials to complain that the US government paid attention to drug violence only when it took the life of a US citizen.