President Barack Obama has been urged to address Cambodia's longstanding human rights issues during a visit this month to the Southeast Asian country, the first by a U.S. president, a human rights group said on Tuesday.

New York-based Human Rights Watch said in a report that more than 300 people have been killed in politically motivated attacks in the past two decades during the rule of Cambodian Prime Minister Hun Sen.

The report suggested senior Cambodian government officials and security forces were involved in a series of serious rights abuses since Cambodia signed peace agreements in 1991 to pave the way for democracy.

It said officials allegedly responsible for extrajudicial killings and other abuses against opposition politicians, security forces, activists and journalists have not been prosecuted, but instead were rewarded.

"The message to Cambodians is that even well-known killers are above the law if they have protection from the country's political and military leaders," said Brad Adams, HRW's Asia director.

Adams said Obama should demand that Hun Sen solve the issue of impunity to bring justice to the victims.

"On his historic first visit to Cambodia, President Obama is uniquely placed to publicly demand that Hun Sen make genuine reforms so the Cambodian people can enjoy the same rights and freedoms that Americans take for granted," Adams said.

Obama is scheduled to attend the East Asia summit in Phnom Penh, Cambodia's capital. It is part of his first overseas trip since being re-elected into office on Nov. 6.