Lalith Kumar Weeraraj and Kugan Muruganathan organised demonstrations by the families of missing people.
They disappeared while organising one such rally in Jaffna, a city central to the civil war which ended in 2009.
A campaign group, Students for Human Rights, handed a letter to the UN’s country chief Subinay Nandy asking that the UN Human Rights Council help investigate their disappearance.
Nuwan Bopage, president of Students for Human Rights, said outside the UN building in Colombo: “We have written to all the authorities but so far they have not even responded to us. We still believe they are in military custody.”
Supporters have maintained that they believe the security forces secretly abducted them, not least because Jaffna is firmly controlled by the military and because the wife of one of the victims says she later spotted their motorcycle inside a police compound.
The army and police deny holding the men. A police spokesman told the BBC a special investigation was under way.
He said there was no evidence of an abduction but asked people to come forward if they had witnessed such an event.
But campaigners said possible witnesses had been threatened or had gone into hiding, and that they had reported seeing five men taking the activists away in an unmarked white van.
The father of one of the missing men, Arumugan Weeraraj, said his son only wanted to help people from northern Sri Lanka find their missing family members.
“Please allow me at least to talk to him,” he said.
It has emerged that the other missing man, Mr Muruganathan, once belonged to the Tamil Tigers but left the separatist group 10 years ago.
Several other people have disappeared in unexplained circumstances since Mr Weeraraj and Mr Muruganathan went missing.
The UN says there are more than 5,000 cases of unsolved enforced disappearance in Sri Lanka. The rate has dropped since the end of the war but it has since risen again.
Since the two human rights workers disappeared there have been no more demonstrations for the families of missing people.
Janatha Muruganathan, the wife of one of the missing men, said outside the UN in Colombo: “Please give my husband back. We are suffering without him. He has done no wrong to anyone. Please help us find him.”
Activists say that during his own campaigns for missing people, Lalith Kumar Weeraraj had visited several places of detention and found there two people who had gone missing.