Obama will be the first US president to visit Cambodia.
"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," said Brad Adams, Asia director at Human Rights Watch.
Adams co-authored a report documenting cases of unsolved killings of activists, opposition politicians and journalists by Cambodian authorities over the past two decades.
According to Human Rights Watch, more than 300 people have been killed in politically motivated attacks since the 1991 Paris Agreements that paved the way for Cambodia to become a democracy.
HRW alleges that numerous Cambodian authorities responsible for killings have never been brought to trial.
"Instead of prosecuting officials responsible for killings and other serious abuses, Prime Minister Hun Sen has promoted and rewarded them," Adams said.//DPA