For Muslims in Saudi Arabia the first day of Ramadan will fall on Wednesday, the kingdom's royal court announced, while the office of Iran's supreme leader said the holy month would begin there on Thursday.
Ramadan, during which hundreds of millions of Muslims around the world observe a fast from sunrise to sunset, traditionally begins with the sighting of the crescent moon.
Many Muslim-majority countries around the world follow Saudi Arabia in determining the start of Ramadan.
"Wednesday is the first day of the blessed month of #Ramadan," the official Saudi Press Agency posted on X Tuesday.
Saudi Arabia's Gulf neighbours Oman, Qatar and the UAE also made similar announcements.
Sunni Islam is the majority religion in Saudi Arabia, while Shiite Islam is the majority faith in Iran.
Iranian supreme leader Ayatollah Ali Khamenei's office said on Tuesday "it is expected -- as reflected in the calendar -- that Thursday will be the first day of the holy month of Ramadan".
Observing the Ramadan fast is one of the five pillars of Islam, requiring believers to abstain from eating, drinking, smoking and sex during daylight hours. Observant Muslims are also encouraged to donate to the poor.
Despite advances in astronomy and the ability to determine the start of Islamic months many years in advance, religious authorities continue to rely on direct visual sighting as the official method for confirming the presence of the crescent moon.
This practice has been followed since the time of the Prophet Mohammed, who linked the beginning of fasting to the sighting of the crescent.
Ahead of the announcement, an astronomical observatory more than 100 kilometres northwest of Riyadh, in the heart of the desert and far from any light pollution, hosted several observers who scanned the sky, according to an AFP journalist.
They were just one of several teams of observers scattered across Saudi Arabia on Tuesday who watched for the appearance of the new crescent moon, signalling the beginning of Ramadan.
Following evening prayers, the teams then file their report to the Supreme Court of Saudi Arabia, which has the authority to announce the start of the holy month.
In Iran, meanwhile, the supreme leader's office announced moon-sighting groups will spread across that country on Wednesday evening to "attempt to observe the crescent marking the start of the holy month of Ramadan".
"Experts predict that the crescent should be easily visible", it added.
© Agence France-Presse