BAKU -- Artur Rasizade, prime minister of Azerbaijan and next-in-line to the country's ailing President Heidar Aliyev, is in the United States for medical treatment, a spokesman for his office said Wednesday.

"The prime minister flew to the U.S. yesterday morning for medical treatment which had been planned for some time. I cannot say when he will be returning," the spokesman told AFP by telephone.

The spokesman said he could not say what Rasizade was being treated for, though there have been reports he is having eye surgery.

Rasizade left the country on Tuesday morning, hours after Aliyev collapsed dramatically on live television fuelling speculation the 79-year-old president may soon step down from office.

Under Azerbaijan's Constitution, the prime minister, who is 67, would become caretaker president for three months until elections have been held.

Many observers believe that Aliyev plans to appoint his 41-year-old son Ilham as prime minister in Rasizade's place, possibly setting the stage for Ilham to succeed him as president.