Jakarta: The world's most populous Muslim country is divided on the start of the holy month of Ramadan, with millions beginning to fast on Friday and others waiting until the government's official start-date of Saturday.

Indonesia's religious affairs minister Suryadharma Ali announced late Thursday that the start of Ramadan would fall on Saturday July 21, after Islamic astronomers dispatched across the country failed to sight the new moon.

"As our observers found no moon sight on Thursday, the Ramadan month will begin on Saturday," Ali said, as quoted by the Jakarta Post newspaper.

But the country's second largest Islamic group Muhammadiyah, which has some 30 million adherents, had earlier announced that according to its astronomical calculations Ramadan begins on Friday.

It is not the first time that the Muhammadiyah has offered a differing opinion on the matter, although the majority of Indonesians are expected to follow the government's official date.

During the holy month, Muslims are required to abstain from food, drink and sex from dawn until dusk.

The fasting is one of the five main obligations for Muslims, who account for more than 90 per cent of Indonesia's 240 million people.

The month traditionally begins with the sighting of the new moon, which varies from country to country.