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Police in northern Colombia are on high alert after a crime gang offered a reward for the killing of police officers.

Officials found leaflets signed by the Urabenos gang offering $500 (£325) for every officer killed, more if they belong to the counter-narcotics force.

It comes a week after counter-narcotics police killed the Urabenos's leader in a raid on his ranch.

The gang controls much of the drug trafficking in northern Colombia.

The police commander of the region, Gen Jose David Guzman, said he had put his officers on alert after capturing a member of the gang as he was allegedly targeting a police officer.

"We don't know if the Urabenos criminal gang will adopt this strategy of attacking police officers, but we've alerted our personnel," he said.

The threat is believed to be in retaliation for the killing last week of Juan de Dios Usuga, the leader of the gang.

He was shot dead by members of the counter-narcotics police as he was celebrating the New Year at his ranch in Necocli, in northern Choco province.

Intimidation

Officers said Mr Usuga, better known as Giovanni, and his bodyguards opened fire on a police helicopter as it landed, killing one of the officers as he was jumping off.

The gang leader was shot dead in the exchange of fire which followed.

Since the killing, the gang has stepped up its activities.

Many communities in northern Antioquia and Cordoba province were paralysed when members of the gang set up armed roadblocks and threatened merchants not to open their shops and businesses earlier last week.

Los Urabenos is one of the groups the Colombian government calls Bacrim, short for bandas criminales (criminal gangs).

Colombian President Juan Manuel Santos has made the fight against the Bacrim one of his government's priorities.

He has offered $2.1m (£1.4m) for information leading to the arrest of the new leader of the Urabenos gang, known as alias Otoniel.

He also announced the creation of a new police force for the city of Santa Marta, one of the Urabenos' strongholds on the Atlantic coast.