(CNN) Two crewmembers died when a tanker connected to an Israeli billionaire was attacked off the coast of Oman, the ship's management firm said Friday.
A Briton and a Romanian were killed amid what the company described as a "suspected piracy incident" onboard the Mercer Street tanker on Thursday.
A US defense official familiar with the details of the incident said the tanker was attacked by an armed drone believed to be operated by Iran.
Another official said the US responded "to an emergency distress call of an apparent UAV style attack," and that the ship was escorted to port by the USS Ronald Reagan aircraft carrier and the USS Mitscher destroyer. US Navy personnel, including explosive experts, went on board the vessel to ensure there was no additional danger, the official said.
The tanker's crew reported that the drone exploded into its super structure on Thursday, the US official said. They also reported an unsuccessful attempted drone attack earlier in the day but said that drone fell into the water.
The US military has long noted Iran has drones that operate by flying into targets and exploding on impact. The incident comes amid heightened tensions between Israel and Iran.
The Mercer Street was in the northern Indian Ocean at the time of the incident, traveling from Dar es Salaam in Tanzania to Fujariah in the United Arab Emirates with no cargo onboard, the ship's management firm said.
It is reported to be sailing under a Liberian flag, according to the maritime tracking website Marine Traffic. The vessel is Japanese-owned and managed by Zodiac Maritime, an international ship management company headquartered in London and led by Israeli shipping magnate Eyal Ofer.
Zodiac also confirmed Friday that the ship was under its own power and moving to a safe location with a US naval escort. For now, the US will not say which port the Mercer will go to.
According to the company, the coordinates of the incident are northeast of Masirah Island, off Oman. An official source at Oman's Maritime Security Center told Oman State News Agency later on Friday that the incident happened outside Omani waters, the agency said.
Oman sent its air jets and a Navy ship to the location of the attack, and Omani officials say they were told by the ship and its crew it would continue sailing without the need for assistance, the agency reported.
Zodiac said it is working with the United Kingdom Maritime Trade Operations (UKMTO) and authorities as investigations continue.
Later on Friday the US State Department said it working was "urgently" to determine what happened.
"The United States extends our sincere condolences to the victims of loved ones and we're also deeply concerned by the reports and are monitoring the situation closely," State Department spokesperson Jalina Porter told reporters in a telephone briefing. "We're urgently working with our partners, our international partners, to establish the facts."
A spokesman from the UK's Ministry of Defence said: "We are aware of reports of an attack on a merchant vessel off the coast of Oman. UK military headquarters in the region are currently conducting investigations."