The plea for global support was made at a conference in Geneva, following the Taliban's takeover last month.
Speaking angrily, the relative of Abdul Hashim, who was allegedly detained and killed by the Taliban, told the BBC: "The Taliban claim to be a government, so they should investigate someone, not just kill them straight away."
"They didn't let men attend Abdul Hashim's funeral," he told the BBC. "Only women were allowed to bury him."
"It is trying to show that it is still an energy superpower, and can retaliate [against] the sanctions Europe has put on it."
Ever since the outbreak of the war in Ukraine, Germany has been trying to get alternative supplies of gas from Norway and the Netherlands.
Mr Rajapaksa had been initially granted a 14-day visa for his stay in Singapore. But that has now been extended another 14 days, Singapore local media reported on Wednesday.