Commonwealth leaders were to wrap up a summit in Sri Lanka Sunday, looking to put on a united front after talks dominated by a bitter dispute over war crimes allegations.

Leaders were expected to issue joint statements on issues such as debt management and funding to combat the threat of climate change to smaller countries before flying out of Colombo.

But the show of unity will do no more than mask the divisions within Commonwealth ranks after three days of talks which have been overshadowed by recriminations over the bloody end in 2009 to Sri Lanka's 37-year war.

The summit was dealt several body blows before it began with the leaders of Canada, India and Mauritius all deciding to stay away to protest Colombo's rights record.

Britain's Prime Minister David Cameron then stole the limelight on the opening day with an historic visit to the war-torn Jaffna region where he met with survivors of a conflict that killed more than 100,000 people.

The summit host, President Mahinda Rajapakse, has had to fend off questions throughout the three-day gathering about his refusal to allow international investigators to probe exactly what happened at the end of the conflict.

According to the United Nations and rights groups, as many as 40,000 civilians may have died as troops loyal to the mainly Sinhalese government routed the Tamil Tiger rebel movement in its last stronghold in Jaffna.

Cameron warned Rajapakse that he would lead a push for an international probe through UN bodies unless an internal Sri Lankan inquiry produces credible results by March.

"Let me be very clear, if an investigation is not completed by March, then I will use our position on the UN Human Rights Council to work with the UN Human Rights Commission and call for a full, credible and independent international inquiry," said Cameron.

Rajapakse later told reporters that Cameron was welcome to his view but added Sri Lanka must be allowed to complete its own investigation in its own time.

"They have to trust us," he said.

"Pressure won't do anything. ... It's much better to wait rather than demand or dictate."

Rajapakse was speaking at a press conference that was meant to be a chance for leaders to outline the progress made on issues of wide concern across the 53-nation bloc.

Only 27 heads of government actually made it to Colombo but Commonwealth Secretary General Kamalesh Sharma denied that the summit had been a flop.

"As far as the outcomes are concerned, you will find at the end of this CHOGM, that it has been very productive, and it has been very meaningful and successful," said Sharma, seated alongside Rajapakse.

One area where they are expected to find common ground is on ways to help smaller countries threatened by climate change access funds that wealthier countries have promised to help mitigate the impact of global warming.

The prime minister of Samoa, one of many Pacific island nations which are threatened by rising waters, said there were no illusions about the dangers of a lack of funds to fight climate change.

"We all know the causes, we all know the solutions, we all know the consequences," said Prime Minister Tuilaepa Sailele Malielegaoi.

"All that is left is the political courage of the most powerful nations to take the action that is necessary to stop climate change," he added.