The announcements came after religious authorities in the two Gulf Arab countries failed to sight the crescent moon on Thursday night, state media reported.
The Sunni mufti of Lebanon, Abdullatif Deryan, also announced that Saturday would be the start of Ramadan.
There was no immediate announcement from other Muslim countries in the Middle East and North Africa.
For the next 29 or 30 days, depending on how long the lunar month will be, the daily fast will last from dawn to sundown - around 15 hours this time of the year in the UAE.
Working hours will be reduced by two hours in most cases, usually starting later and ending earlier. Each evening, a calm will prevail as most residents rest at home in the build up to iftar, the sunset meal taken to end the day's fast.
They break the fast with a meal known as iftar and before dawn they have a second opportunity to eat and drink during suhur.
Ramadan is followed by the feast of Eid ul-Fitr.
It starts the day after Ramadan ends and lasts for three days.
Eid al-Fitr includes special prayers and meals with friends and relatives, and gifts are often exchanged.
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