Caroline Gluck reports from Phnom Penh

In a statement, the US State Department warned the Cambodian Government that it could jeopardize relations with the United States unless the Khmer Rouge leaders were held accountable for their actions.

The comments came after Khieu Samphan made his first public apology for the "killing fields" genocide in Cambodia which left up to two million people dead in the 1970s.

US State Department spokesman James Foley said: "Former Khmer rouge leaders Nuon Chea and Khieu Samphan are now in Phnom Penh.

"As leaders of the regime responsible for the deaths of up to two million people, they should be held accountable for their actions before an appropriate tribunal."

'I am very sorry'

Khieu Samphan: "We have to forget the past"

Khieu Samphan spoke on Tuesday after he and Nuon Chea were welcomed to talks in the capital, Phnom Penh, by Cambodian Prime Minister Hun Sen. Both men defected to government forces last week.

"Yes - sorry, sorry, sorry, I am very sorry," he said when asked if he felt remorse for his role in the Khmer Rouge's four years in power.

"Actually we are very sorry, not just for the lives of people, but also for the lives of animals that suffered in the war," he said, adding that "history should remain history".

He told reporters he understood that many Cambodians were resentful at leaders of the guerrilla movement since many of their family members had lost their lives. But he appealed to them to look forward, saying that the country still faced many problems.

"It is time to let bygones be bygones" he said.

Cambodian exile Dana Tep tells BBC World why she wants those responsible for genocide brought to justice

The last time he visited the Cambodian capital in 1991, just after the signing of the Paris peace accords formally ending the country's civil war, he came under attack from an angry crowd.

During talks with Hun Sen, Khieu Samphan, many years the international face of the shadowy Khmer Rouge, spoke of the need to forget the past in the interests of national reconciliation.

On Monday, the Cambodian premier made clear that he opposed international calls for the defectors to be tried and said Cambodia should be allowed to decide for itself how to deal with the two men.

"Those who once led a war have requested to rejoin and live in the national society" he said.

"So what we will welcome them with are not guns, bullets, prison, or handcuffs, but a bouquet of flowers hailing their spirit of national reunification."

He said the government's policy now was one of reconciliation and any trial would be divisive and mean a return to civil war.

'No amnesty'

Prime Minister Hun Sen (in Khmer): "The two have returned as normal citizens"

But Cambodia's Secretary of State for Information, Khieu Kanyarith, told the BBC that under the International Convention on Genocide, the government had no powers to "grant amnesties to the perpetrators of genocide".

Secretary of State for Information, Khieu Kanyarith: "We are not saying we have closed the case"

The UN special rapporteur on Cambodia, Thomas Hammarberg, told the BBC he hoped there was still an openness among political leaders to proceed with previous pledges to bring Khmer Rouge leaders to justice.