PHNOM PENH : Police forces loyal to Cambodian Prime Minister Hun Sen forced the acting head of state Chea Sim out of the country in a purge of the ruling party, diplomatic sources told AFP.

"They used the police forces under (national police chief) Hok Lundy to surround the house of Chea Sim" in the early hours of the morning, one of the sources told AFP on condition of anonymity.

"Negotiations occurred and the deal was that he had to leave the country with General Hok Lundy escorting him out. They forced him out of the country," he added.

The removal of Chea Sim, who is also president of the long-ruling Cambodian People's Party (CPP), came after he refused to sign a controversial bill into law aimed at resolving a year-long political crisis.

Hun Sen and Chea Sim head different factions in the party.

King Norodom Sihanouk, who is currently in North Korea in self-exile waiting out the crisis, had already declined to give his royal approval to the bill and instead asked Chea Sim to make a decision using "his conscience".

The CPP, an ex-communist party that had previously had strong internal discipline, denied any rift.

"He was not forced out. He decided himself to go for medical treatment," CPP spokesman Khieu Kanharith told AFP, confirming that Hok Lundy was in Thailand along with other party officials.

He admitted forces had descended near Chea Sim's house, which is next to the senate that he heads, but said they had done so at the request of the Constitutional Council which feared there had been a security breach in the area.

The bill was instead signed by royalist senator Nheik Bun Chhay.

A diplomat told AFP that he believed a government would still be formed but under tense circumstances.

"The secret division in the CPP is now an open secret. I think the show will go on but it will be more tense and it won't be good for the country," he told AFP.

Another observer said the departure of Chea Sim would conveniently allow the factions to continue presenting a united front although the latest developments made it clear Hun Sen was truly in charge.

A letter from Chea Sim to the king posted on his website late Tuesday asked for his permission to take one to two weeks' medical leave in Thailand, to which the king answered positively.

Some areas in Phnom Penh experienced blackouts and water cuts Tuesday. Traffic was relatively light and some Cambodians closed their shops and reported friends staying home, fearing unrest which has flared suddenly in the past in Cambodia. Security presence on the streets was normal.

In neighbouring Thailand, Foreign Minister Surakiart Sathirathai said he was closely monitoring the situation.

"There have been no reports of any kind of violence but there has been a significant build up of political tension," he told reporters.

Opposition leader Sam Rainsy labelled Chea Sim's departure a "party coup d'etat" and told AFP he had left the kingdom bound for the United States where he would try to rally international support against the new government.

"It is becoming more and more illegal," he said, referring to the manoeuvrings that have taken place in order to secure a June 30 coalition pact between the CPP and their coalition partner, the royalist FUNCINPEC party.

Khieu Kanharith said the parliament could still be convened on Thursday.

The controversial law will now allow Hun Sen and royalist leader Prince Norodom Ranariddh to be jointly elected to their posts in the new administration at the next session.

Fear and mistrust between and among their parties prompted the deal that will see them voted in with a show of hands.

- AFP