|Pascual resigned after a WikiLeaks document revealed his criticism of Mexico's drug war [Reuters]
Carlos Pascual, the US ambassador to Mexico, has resigned after a dispute with Felipe Calderon, the Mexican president, over the handling of the war against drug gangs.
Saturday's announcement came as Barack Obama, the US president, began a five-day trip to Latin America, where he is visiting El Salvador, Brazil and Chile, to shore up ties with the region.
The diplomatic spat was triggered after state department documents published by WikiLeaks showed Pascual criticising Mexico for its lack of co-ordination in operations targeting cartel leaders.
Calderon hit back in a newspaper interview on February 22, saying Pascual had shown "ignorance" and distorted what was happening in the country.
He also said US security forces failed to co-ordinate their own efforts and saw each other as "rivals".
Calderon is facing increasing pressure in Mexico over his security strategy as the toll from drug violence has climbed to more than 36,000 since he took office in late 2006.
In a visit to Washington earlier this month, Calderon reportedly requested that Pascual be removed from his post.
Hillary Clinton, the US secretary of state, said on Saturday Pascual decided to resign "to avert issues raised by President Calderon that could distract from the important business of advancing our bilateral interests".
Mexico and the US trade more than $1bn a day across their long border and in recent years stepped up intelligence sharing in operations to bring down major drug traffickers.
But the alliance has been strained by the public dispute between Calderon and Pascual and US failure to stop weapons smuggling into Mexico.
A decision to allow unmanned surveillance drones fly over Mexican territory has drawn criticism, with opposition politicians saying it violates Mexico's sovereignty.
The killing of a US immigration official in a suspected drug-cartel ambush last month also raised tensions.