Sri Lanka is among the most dangerous places on earth for humanitarian workers, the UN's aid chief says, calling on the government to probe civil war abuses and consider an international rights monitoring mission.

Aid agencies say 34 humanitarian staff have been killed in Sri Lanka since January 2006, including 17 local staff of Action Contre La Faim shot dead in the restive northeast a year ago in a massacre Nordic truce monitors blamed on security forces.

"There is a concern ... about the safety of humanitarian workers themselves and the record here is one of the worst in the world from that point of view," John Holmes, UN Under Secretary General for Humanitarian Affairs, told Reuters in an interview late on Wednesday during a visit to Sri Lanka.

"Democratically elected governments are judged by higher standards than other people, so they need to address these concerns, they need to look into them, they need to investigate allegations that are made."

President Mahinda Rajapaksa's government denies security forces have been involved in rights abuses and says a presidential commission is probing the allegations. The government has also rejected calls for a United Nations rights monitoring mission.

"It would be good for Sri Lanka's international image that there is that kind of monitoring to demonstrate, which the government say, they have nothing to hide in this area," Holmes said.

"Of course it's not just the government, the LTTE (Tigers) are guilty of many abuses, child abduction, child recruitment of soldiers are very serious problems with the LTTE but also with the Karuna faction in the east of the country," he added, referring to breakaway rebels UNICEF says recruit children.

He said the government had agreed to disarm the Karuna faction, which has said it will lay down arms once the Liberation Tigers of Tamil Eelam no longer represent a threat to them.

Nearly 70,000 people have been killed in the conflict in Sri Lanka since 1983 - around 4,500 in the last year alone.

KILLINGS, ABUSES

Holmes said allegations of rights abuse by government forces such as those made in a report by the U.S.-based Human Rights Watch this week must be dealt with seriously.

"They are not frivolous organisations, they don't make frivolous charges. They need to be looked at seriously."

The government had dismissed the report saying its allegations of unlawful killings and disappearances against the security forces were unfounded and based on outdated and incorrect information.

Holmes said he had positive and frank discussions with government officials, and had been reassured that abuses would be looked into.

He visited the island's army-held Jaffna peninsula in the far north, cut off from the rest of the island behind rebel lines, and a captured former rebel stronghold in the east, where the government has resettled thousands of civilians.

Areas ravaged by Sri Lanka's protracted conflict are highly militarised, and that is hampering the delivery of aid.

The government accused some aid agencies of helping the Tigers after finding equipment and tarpaulins stamped with agency logos in captured Tiger camps.

"We need the ability to operate without being regarded as hostile by any side, without being regarded as somehow suspicious," Holmes said. "We are simply trying to help people live their normal lives."