TIJUANA, Mexico (AFP) - A 14-year-old boy was arrested in Mexico for killing a man after he was allegedly contacted on Facebook and offered US$1,900 (S$2,600) to commit the crime, authorities said.

Prosecutors said the minor was caught on Saturday as he tried to flee the scene of the crime in the northwestern city of Tijuana, which borders California.

"The child said that he was contacted by a person via Facebook," Miguel Angel Guerrero, a special investigations coordinator in the Baja California state prosecutor's office, told reporters late Tuesday.

The person offered the boy 31,000 pesos (S$2,600) to kill someone, Guerrero said.

The child told investigators that he agreed to meet the person at one of the city's main roads.

A taxi took him to a neighbourhood known for its bars and drug dealing where "they gave him a weapon and they pointed to the person he had to kill," Guerrero said.

The boy got out of the taxi, walked over to the 35-year-old victim and shot him in the head with a .40-calibre gun, the official said.

City police caught the child as he fled and he was handed over to a juvenile crimes court.

Authorities released a picture of the boy with his face blurred out. He has dark hair and wears a white shirt with red and blue stripes.

Mexican drug cartels are known to recruit minors to work as "halcones" (hawks), or lookouts, to warn them about the movements of security forces or suspicious people.

But there have been some cases of children committing violent crimes.

In March, a 13-year-old boy was arrested in the northern industrial hub of Monterrey in possession of an AK-47 assault rifle, a handgun, bullets and drugs.

In December 2010, authorities arrested a US-born 14-year-old who was later convicted of working as a hitman for a drug gang. He admitted to killing four people, in a case that shocked Mexico.

The boy, known as "El Ponchis," was released in 2013 at age 17 and flew to Texas to be reunited with his mother.