WASHINGTON — Stepping up its involvement in Mexico’s drug war, the Obama administration has begun sending drones deep into Mexican territory to gather intelligence that helps locate major traffickers and follow their networks, according to American and Mexican officials.

The Pentagon began flying high-altitude, unarmed drones over Mexican skies last month, American military officials said, in hopes of collecting information to turn over to Mexican law enforcement agencies. Other administration officials said a Homeland Security drone helped Mexican authorities find several suspects linked to the Feb. 15 killing of Jaime Zapata, a United States Immigration and Customs EnforcementImmigration agent.

President Obama and his Mexican counterpart, Felipe Calderón, formally 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 in the past four years, such an agreement would have been all but unthinkable, they said.