The Sri Lankan authorities must act to ensure the safety of two female editors at a national newspaper who received death threats last week, Amnesty International said.
The organization also called for an immediate investigation into the threats, received by Frederica Jansz, Editor-in-Chief of the Sunday Leader, and the newspaper's News Editor, Munza Mushataq.
The threats, which were written in red ink, were delivered by post to the newspaper on 22 October.
The founder and former Editor-in-Chief of the newspaper, Lasantha Wickrematunge, was murdered in January three weeks after receiving a similar death threat also written in red ink.
No one has yet been prosecuted for his murder.
The most recent threats relate to the coverage by the Sunday Leader of a video, broadcast on UK TV station Channel Four in August, which allegedly showed Sri Lankan soldiers executing Tamil prisoners.
The Sri Lankan government has stated that the video had been faked, but on 18 October the Sunday Leader printed an article on its front page, highlighting a report that an analysis of the video had concluded that the footage had not been tampered with or edited.
The threatening letters were postmarked 21 October, just three days after the newspaper ran its controversial story. Both letters included text saying "if you write anymore, we will kill you, [and] slice you into pieces".
The journalists reported the threat to Sri Lanka's Inspector General of Police, and also to local police in the capital, Colombo. However, no action has yet been taken by the authorities.
In September, Dileesha Abeysundera, who works for the Sinhala-language edition of the Sunday Leader and also campaigns for greater press freedom in Sri Lanka, was threatened.
There have been numerous serious attacks on the staff of the Sunday Leader and its publishers. Its offices have been burnt down, bombed and sealed several times.
Over the past three years, numerous journalists have been detained in Sri Lanka while others have fled the country. At least 14 media workers have been killed. Investigations have not resulted in prosecutions.