The Colombian army says it's sent in large reinforcements to a remote police base in the east of the country which is under seige from left-wing rebels.

At least four policemen have been killed and seven wounded in the attack, which is being blamed on the country's largest rebel group, the Revolutionary Armed Forces of Colombia, or FARC.

Communication with the police base, near the town of Mitu in Vaupes province has been cut, and unconfirmed reports put the number of police killed at about sixty.

The attack comes days before the controversial withdrawal by the armed forces from large areas of Colombia.

Correspondents say many top army officers have privately criticised the withdrawal as further evidence that the government is making too many concessions to the rebels ahead of formal peace talks.