Authorities are investigating suspected attacks on two tankers in the Gulf of ...

US officials have not ruled out the possibility that the tankers involved in the Gulf of Oman incident Thursday were attacked by a projectile or hit an underwater mine.

"The US at this hour has not ruled out [that] the ships may have hit a mine in the water, or were attacked by a projectile. They are trying to determine the cause," a US defense official told CNN.

A US P8 Poseidon maritime patrol aircraft is conducting surveillance in the area, according to the same source.

The guided missile destroyer USS Bainbridge responded to the incident after the US Fifth Fleet -- responsible for US naval forces in the area -- received two separate distress calls on Thursday morning.

The US defense official said the destroyer took on board 21 mariners from the Kokuka Courageous tanker, who were originally rescued by a tug boat.

The two tankers were targeted Thursday morning in the Gulf of Oman, the same waterway where four commercial ships were attacked last month.

The attacks have ratcheted up existing tensions in the region, where Iran has long been at loggerheads with Saudi Arabia and the UAE -- most recently over the civil war in Yemen.

Thursday's suspected attacks on two tankers in the Gulf of Oman come a month after four commercial ships were hit in the same waterway, heightening tensions in a region already on edge.

But these reported attacks appear to be even more aggressive. Here's the difference between the two incidents.

May 12 incident

In the incident on May 12, four ships were at anchor in the UAE port of Fujairah, a few kilometers from the coast, when they were apparently hit by mines or improvised explosive devices likely attached to their hulls overnight. The attacks caused no injuries and no evacuation. They were, essentially, pin-prick strikes, a subtle message.

The US and Saudi Arabia suspect Iran was behind those attacks -- though no evidence of its involvement has been presented. Tehran denied any involvement, and precisely who carried out the attack is still under investigation.

Today's incident

The two tankers involved in today's suspected attacks were some 70 kilometers from the UAE, closer to the Iranian coast.

One of them was hit above the water line by what witnesses described as “some sort of shell," according to an official from the firm that owns the boat. The other ship caught on fire following an explosion. The crews of both boats were evacuated.

There has been no assigning of blame thus far today, but the volume has been turned up.

Kokuka Courageous

A file image of the Kokuka Courageous, a tanker reportedly "attacked" twice with "some sort of shell," in the Gulf of Oman on Thursday.

Front Altair

A file photograph of the Front Altair, an oil tanker struck by an explosion followed by a fire in the Gulf of Oman on Thursday.

Iran's Foreign Minister Mohammad Javad Zarif has said "suspicious doesn't begin to describe" the incident involving two tankers in the Gulf of Oman this morning.

Zarif said news of the reported attacks on Japan-linked tankers broke while Japan's Prime Minister Shinzo Abe was meeting Iran's Supreme Leader Ayatollah Ali Khamenei.

One of the two tankers involved in the incident, Kokuka Courageous, is owned by Japan-based company Kokuka Sangyo. According to Japan's Ministry of Economy, Trade and Industry both tankers were carrying "Japan-related cargo."

Abe is currently in Tehran on a visit viewed as an attempt to mediate US-Iran tensions -- it is also the first trip by a Japanese premier since the 1979 Islamic Revolution.

The UK government has said it is “deeply concerned” about an incident involving two tankers in the Gulf of Oman.

“We are deeply concerned by reports of explosions and fires on vessels in the Strait of Hormuz,” a government spokesperson told CNN on Thursday.

“We are in contact with local authorities and partners in the region.”

A fire broke out after an explosion on board the Front Altair, one of two ships involved in the security incident in the Gulf of Oman, according to International Tanker Management (ITM), which manages the tanker.

An investigation is underway into the cause of the explosion, which is as yet unknown, and ITM is working on plans to salvage the vessel.

All 23 crew members, who were unharmed, were picked up by another vessel, according to ITM. The front Altair was in the southern end of the Strait of Hormuz when the incident occurred.