COLOMBO (REUTERS) - A rights group documenting alleged abuses in Sri Lanka has filed a criminal complaint with Singapore's Attorney-General, seeking the arrest of former president Gotabaya Rajapaksa for his role in the South Asian nation's decades-long civil war.

The International Truth and Justice Project (ITJP) said Mr Rajapaksa committed grave breaches of the Geneva Conventions during the civil war in 2009 when he was the country's defence chief, according to a copy of the complaint seen by Reuters.

The South Africa-based ITJP argued that based on universal jurisdiction, the alleged abuses were subject to prosecution in Singapore, where he fled after months of unrest over his country's economic crisis.

Mr Rajapaksa submitted his resignation in Singapore, a day after fleeing on July 13. Anti-government protesters had stormed the offices and official residences of the president and the prime minister.

"The criminal complaint that has been filed is (based on) verifiable information on both the crimes that have been committed, but also on evidence really linking the individual in question, who is now in Singapore," Ms Alexandra Lily Kather, one of the lawyers who drafted the complaint, told Reuters by telephone from Berlin.

"Singapore really has a unique opportunity with this complaint, with its own law and with its own policy, to speak truth to power."

Mr Rajapaksa could not be reached for comment through Sri Lanka's High Commission in Singapore. He has previously strenuously denied allegations he was responsible for rights abuses during the war.

In response to questions from Reuters, a spokesman for the Attorney-General’s Chambers said it had received a letter from the ITJP on July 23.

“We are not able to comment further on this matter,” said the spokesman.

The country's Foreign Ministry has said Mr Rajapaksa entered the South-east Asian city-state on a private visit and had not sought or been granted asylum.

Professor Shubhankar Dam at the University of Portsmouth School of Law in Britain, who has taught in Singapore, said that while its courts were able to try alleged war crimes, genocide and torture, it has repeatedly stated that such jurisdiction should be invoked only as a last resort.

"While neutrality is not officially enshrined in Singapore's foreign policy, it has long cultivated a form of even-handedness," he said.

"Any decision to prosecute a former foreign head of state has to be balanced against its foreign policy objectives."

Sri Lanka ended a 25-year civil war between separatist insurgents from the ethnic Tamil minority and government forces in 2009. Rights groups accused both sides of abuses during the war.

The ITJP assisted in two civil lawsuits against Mr Rajapaksa, proceedings for one of which were served in a California parking space in 2019. He was a United States citizen at the time.

Both cases were withdrawn after Mr Rajapaksa was granted diplomatic immunity upon becoming president later that year.