Speaking to reporters Khieu Samphan's daughter, Khieu Rattana, said her father "asked authorities for permission to leave for treatment, but they refused".
Cambodia's prime minister, Hun Sen, said he had ordered a helicopter to fetch the former Khmer Rouge president from the border town of Pailin and fly him to the capital after hearing reports that he had fallen ill.
"If he dies, people will blame the government," Hun Sen said.
|The former president has portrayed himself |
as a virtual prisoner of the Khmer Rouge [AP]
Critics of the UN-backed tribunal say the process has been delayed too long and suspects may die before ever facing trial.
Pol Pot, the leader of the Khmer Rouge, died in 1998, while his military chief, Ta Mok, died in 2006.
Chea Chandin, Pailin's deputy police chief, said Khieu Samphan was not under arrest.
"We are preparing to send him for medical treatment. This does not mean he is under house arrest or has been arrested," he told Reuters.
Khieu Samphan, believed to be 76, was expected to arrive in the capital later on Wednesday.
His wife said he was recovering and was now able to speak more clearly, walk with some help and eat food after receiving home care from doctors.
"He was suffering from high blood pressure but he's back to normal now," she told AFP.
Prepared for trial
She said her husband was ready to face the tribunal.
"He is not worried about being arrested. He has been ready to face the tribunal for a long time now," she said, speaking from Pailin, the former Khmer Rouge stronghold where Khieu Samphan has lived since surrendering to the government in 1998.
Khieu Rattana said her father knew his arrest was imminent, but added that she did not think this caused his condition, which came just a day after two of his former colleagues were arrested by the tribunal.
Earlier this week Ieng Sary, the former Khmer Rouge foreign minister, and his wife Ieng Thirith, were arrested in a dawn police raid on their Phnom Penh home.
On Tuesday both were formally charged by the UN tribunal with crimes against humanity.
The Khmer Rouge is blamed for the deaths of about 2 million people during their rule between 1975 and 1979.
Most died from starvation, disease, overwork and execution.
The UN-assisted tribunal was created last year after seven years of contentious negotiations between the United Nations and Cambodia.
Along with Ieng Sary and his wife, Nuon Chea, the former Khmer Rouge ideologist, and Kaing Khek Lev, better known as Duch, the former head of the notorious S-21 or Tuol Sleng prison, have also been arrested.
However, no Khmer Rouge leader has been tried and trials are expected to begin only next year.
According to prosecutors' papers, the final of five suspects they seek to charge is Khieu Samphan.
French-educated Khieu Samphan, whose Marxist theories influenced the communist Khmer Rouge's policies, published a book portraying himself as a virtual prisoner of the Khmer Rouge during its reign.