Suspected rebels shot dead a police officer and two constables on Monday night near the township of Thanamalvila, police officials said.
They had been deployed in the area to protect civilians from guerrilla attacks.
"We are sending several senior officers together with reinforcements to bolster security in the area," a police official said, adding that several thousand additional security personnel had been deployed.
The authorities were also recruiting more men to guard vulnerable areas while police stations in other areas have been asked to release constables to beef up security in the south, the official said.
The military has accused the Liberation Tigers of Tamil Eelam (LTTE) of targeting civilians in the south as government troops stepped up attacks against the ethnic Tamil rebels in the north.
In heavy fighting in the northern regions where the Tigers maintain a de facto mini-state, the defence ministry reported a death toll of 51 on Sunday and Monday, including two soldiers.