The bitumen tanker MV Asphalt Princess is heading into the congested approach to the Strait of Hormuz, it said.
It is not clear who has seized the ship but analysts says Iranian forces will be suspected.
Iran's Revolutionary Guards dismissed the reports as a pretext for "hostile action" against Tehran.
The incident comes less than a week after an oil tanker operated by an Israeli-owned company was attacked by a drone off Oman, killing two security guards - one British and the other Romanian.
The US, UK and Israel blamed Iran for the attack - a claim it strongly denies.
BBC Security Correspondent Frank Gardner says the MV Asphalt Princess is owned by a Dubai-based company that had one of its ships hijacked two years ago by revolutionary guards.
He says that, according to reports, up to nine armed men boarded the MV Asphalt Princess as it neared the entrance to the Strait of Hormuz, through which about a fifth of the world's maritime oil supplies passes.
Earlier, the United Kingdom Maritime Trade Operations (UKMTO) advised shipping near Fujairah in the Gulf of Oman to exercise extreme caution.
A spokesperson for the UK Foreign Office said it was urgently investigating reports of an incident on a vessel off the UAE coast.
The White House called the reports "deeply disturbing".
UKMTO had initially warned ships of an incident before declaring a "potential hijack" hours later.
Earlier in the day, four oil tankers reported that they were "not under command", which usually means that a vessel has lost power and cannot steer. One of the ships later began moving, AP news agency reports.
An Oman Air Force marine patrol aircraft was seen flying over the sea at the same time, according to data from FlightRadar24.com.
Iran's foreign ministry said reports involving several ships on Tuesday were "suspicious" and warned against any effort to "create a false atmosphere" against Tehran.