Criminal gangs have shot dead another police chief in Mexico City.

Esteban Robles was shot from a passing truck by four gunmen and died on his way to hospital.

Mr Robles, who was head of a department that specialises in abduction cases, has become the fourth such killing in just over a week.

On Thursday, federal police chief Edgar Millan was gunned down at his home.

Two other senior police officers were also killed in recent attacks.

Eighteen months ago, the Mexican government launched a large-scale offensive against the country's drug cartels, which saw thousands of police officers and soldiers being deployed to hunt down the criminals.