Cambodian police vowed to arrest opposition leader Sam Rainsy if he returns to the kingdom as planned on Monday night, in a move condemned as a political attack by the country's strongman prime minister on his main rival.

A court issued an arrest warrant for Rainsy last week over an unserved two-year sentence for defamation, a day after Hun Sen threatened him with a separate legal action for comments urging the premier to move towards a peaceful exit from office.

Hun Sen -- who has ruled Cambodia for more than three decades -- is frequently accused of influencing the courts to tie-up his opponents, a tactic used several times since a 2013 election that Rainsy says was rigged by the ruling Cambodian People's Party.

Cambodian Prime Minister Hun Sen has held office for more than three decades Alain Jocard, AFP

Rainsy, who is currently visiting South Korea, was sentenced in 2011 for accusing the foreign minister of being a former member of the brutal Khmer Rouge, which ruled Cambodia from 1975-1979.

He is due to arrive at Phnom Penh's airport at around 10:30pm (1530 GMT) and police say they will arrest him when he sets foot in the country.

"We must arrest him. It is the duty of police. We must do it after the court ordered his arrest," Kirt Chantharith, a spokesman for the national police, told AFP.

The order to arrest Rainsy was confirmed by Interior Minister Sar Kheng, while the spokesman for the parliament said the opposition chief automatically lost his status as a lawmaker after the warrant was issued -- and with it immunity from arrest.

"When the warrant was issued, he lost all of these status," Chheang Vun told reporters.

Cmabodian opposition leader Sam Rainsy's recent comments urging the international community to pressure Hun Sen for full democratisation were made during his trip to Japan last week in comments inspired by the historic vote in Myanmar Yoshikazu Tsuno, AFP/File

In a video posted on his Facebook page over the weekend, Rainsy said he would defy the warrant and return to Cambodia as planned, setting up a potential showdown late on Monday.

"Cambodia is my birthplace, my mother country, I must return back in order to rescue our nation. So there is no change to my schedule. If I must die, let it be," Rainsy said.

The United States has led calls for the warrant to be withdrawn, suggesting there has been "political influence over the judicial process", while rights groups say the move matches a pattern of repression by Hun Sen.

The opposition chief is Hun Sen's main rival and was in self-exile at the time of the sentence as he also faced a string of other convictions he claimed were trumped-up.

Rainsy only returned to Cambodia ahead of the flawed 2013 elections after receiving a royal pardon for the sentences against him. According to Human Rights Watch, the 2011 defamation case was not mentioned in the pardon.

Rainsy's recent comments urging the international community to pressure Hun Sen for full democratisation were made during his trip to Japan last week in comments inspired by the historic vote in Myanmar.