It said remains of home-made explosives had been found on the hull of the M Star, which was damaged last week while travelling from Qatar to Japan.
Two days ago, an al-Qaeda-linked group said it was responsible.
Officials previously said the ship may have been involved in a collision.
The crew of the ship reported an explosion shortly after midnight on Wednesday last week. One person was injured.
Analysis
This was an amateur operation using crude homemade explosives which did comparatively little damage and caused no fatalities.
It is in marked contrast to the well-planned al-Qaeda attack on the USS Cole in Aden in 2000 which blasted a hole in the warship and killed 17 US sailors, and a further al-Qaeda attack on the tanker Limburg off Yemen in 2002.
A group calling itself the Abdullah Azzam Brigades has issued an online statement claiming responsibility, but experts say this is a name of convenience rather than an actual organisation.
Their stated aim is the release of an ageing Egyptian cleric from a US jail for his part in the 1993 World Trade Center attack - not a cause that has widespread resonance these days.
Still, for a ship to be struck in the narrow waterway between Oman and Iran, through which 40% of the world's crude oil is exported, is a worrying development for merchant shipping. Security in the Gulf is likely to be reviewed as a result.
The BBC's security correspondent Nick Childs says the news is likely to raise concerns about security for shipping in the strait, a vital strategic waterway for oil supplies from the Gulf.
Minor damage
"An examination carried out by specialised teams had confirmed that the tanker had been the subject of a terrorist attack," the news agency said, quoting an unidentified coastguard source.
"UAE explosives experts who collected and examined samples found a dent on the starboard side above the water line and remains of home-made explosives on the hull," it said.
"Probably the tanker had encountered a terrorist attack from a boat loaded with explosives," the source was quoted as saying.
The ship's owner, Mitsui OSK, said it could not confirm the reports. Japanese Foreign Minister Katsuya Okada said the government was aware of the report but was yet to confirm its details, the Agence France Presse news agency said.
Previous theories about the cause ranged from a large wave to a US nuclear submarine. Part of the mystery surrounding the incident was the relatively minor damage to the tanker, which did not seem to conform to any particular explanation.
Map of Strait of Hormuz showing location of M Star at time of attack
A statement signed by a "battalion" of the Abdullah Azzam Brigades, which is linked to al-Qaeda, said one of its suicide bombers was responsible for the blast.
The claim, posted on a website used by Islamist militants, could not be independently verified, and the group has made false claims in the past.
The French-operated tanker Limburg was badly damaged off Yemen in an attack in 2002, as was the US Navy warship Cole in an incident in the Yemeni port of Aden in 2000, in which 17 US sailors were killed.
source bbc news
