Human Rights Watch (HRW) alleges more than 300 people have been killed in politically motivated attacks in over two decades of authoritarian rule by Cambodian Prime Minister Hun Sen.

In a new report "'Tell Them That I Want to Kill Them': Two Decades of Impunity in Hun Sen's Cambodia", the watchdog implicates several senior officials and military officers in murders which it says were never fully investigated.

The cases include a 1997 grenade attack that killed 16 people and which according to HRW was linked by the UN and the US Federal Bureau of Investigation to Hun Sen's personal bodyguard unit.

"Cambodian Prime Minister Hun Sen's violent and authoritarian rule... has resulted in countless killings and other serious abuses that have gone unpunished," the New York-based group said in a statement.

The newly re-elected Obama will be joining regional leaders at the East Asia summit in Phnom Penh from Monday.

HRW urged him to demand "systematic reforms and an end to impunity for abusive officials" when he meets Hun Sen, who has been in power since 1985 and has vowed to rule until he is 90.

"If Obama doesn't speak out about human rights abuses... his visit will be seen by the government as an endorsement and deepen the sense of inviolability," the group's deputy Asia director Phil Robertson told AFP.

The Cambodian government has come under increasing pressure from rights groups and the UN for its crackdown on dissidents and protesters.

Many of the cases are linked to land disputes, now seen as the country's most pressing human rights issue.

This year has been particularly bloody with the murders of a well-known environmental activist, a journalist who exposed illegal logging and a 14-year-old girl who was shot by security forces in a land battle.

"Obama should state very clearly that the Americans know what's going on, they don't approve of it and they expect serious reforms," said Robertson.

A dozen US senators and members of Congress last month also urged Obama to speak out over Cambodia's "deteriorating human rights situation".

Cambodian government spokesman Phay Siphan, who had not seen HRW's report, told AFP his nation "respects all the issues raised" by rights groups "to help us improve ourselves".

But he warned against groups trying to "take advantage" of Obama's visit.