President Uribe came to office vowing to crack down on the rebels

The President of Colombia, Alvaro Uribe, has told army generals to resign if they do not get results in their fight against left-wing rebels.

Mr Uribe's statement came as a top American military official, General Richard Myers, arrived in Colombia to see for himself the impact of billions of dollars of US military aid.

Correspondents say the visit is also designed to show support for Mr Uribe's campaign to suppress rebels.

Car bombs have killed at least eight people and injured 25 in the last four days.

Drugs fight

Mr Uribe marked his impatience with the wave of rebel attacks with a blunt message for the military commanders.

"Brigade and battalion commanders who are doing well - great. But those that are doing poorly and not getting results should hand in their resignations," he told reporters.

The president is due to hold talks with General Myers later on Tuesday.

In one attack on Sunday night, one person was killed and at least 16 injured in an explosion in San Martin, 110 kilometres (70 miles) south of Bogota.

The authorities have blamed the attacks on Colombia's largest rebel group, Farc (Revolutionary Armed Forces of Colombia).

The BBC's Jeremy McDermott in Medellin says the rebels are seeking to undermine the tough security policy of Mr Uribe, who marks his first year in office this month.

The US has given about $3bn in mostly military aid to Colombia over the last three years.

The US programme is aimed at helping Colombia to eradicate the production of coca - the raw ingredient of cocaine - by aerial spraying.

US special forces have also trained Colombian troops in their fight to end the 39-year insurgency.