Amnesty International said on Thursday the National Coalition of Syrian opposition forces that has won widespread recognition must work to free Ukrainian journalist Ankhar Kotchneva, who was kidnapped two months ago.
The "release of journalist must be the first test of the opposition coalition," a statement said.
"Syrian opposition leaders need to secure Ankhar Kochneva's safe release immediately. Hostage-taking during armed conflict is a war crime," said Ann Harrison, deputy director of Amnesty's Middle East and North Africa Programme.
"There are increasing reports of opposition forces carrying out horrific abuses of captured government soldiers, journalists, and some other civilians," Amnesty said.
"The [National] Coalition must condemn these grave abuses in the strongest possible terms, and do its utmost to prevent them."
Amnesty urged the coalition "to act now to ensure that its allied armed groups adhere strictly to their obligations under international humanitarian law and commit to upholding human rights standards."
Arab and Western states which support the ouster of President Bashar al-Assad on Wednesday recognized the National Coalition as the Syrians' sole representative, a day after US President Barack Obama endorsed the group.
Also on Wednesday, Kotchneva's mother issued an emotional appeal for her release, on the eve of the expiry of a ransom deadline that appeared to hold her life on the line.
Her abductors, who claim to belong to the rebel Free Syrian Army, have said they will execute the young woman on Thursday unless they receive a ransom of $50 million (38 million euros), according to Ukrainian media.
Addressing the captors in a video posted online, a tearful Lioudmila Kotchneva said: "You also have mothers and children. Please, why commit a sin? I beg you, set her free."
Kotchneva was working in Syria as an assistant and interpreter for Russian media when she was kidnapped in early October.
In another video posted online this month, the journalist said she had worked for Russian and Syrian intelligence in remarks she is thought to have made under duress.