ABC News chief Washington correspondent Jonathan Karl was on the phone with President Donald Trump just as CENTCOM announced the retaliatory attack against Iran.

"I think it's very important to respond. They shot down a helicopter, and we are responding as we speak," Trump said.

"I believe in responding in a strong manner. I always have throughout my life. And we have a deal that was very good, and probably will [still be]," the president continued. "This is a response to what they did they did with our helicopter last night, and I believe the response should be very strong, very powerful, and that's what this one is."

As the call ended, CENTCOM announced the attack.

U.S. forces have begun launching "self-defense" strikes in response to Iran's attack on a U.S. Army Apache helicopter, U.S. Central Command said.

The strikes began at 5 p.m. ET, it said.

The U.S. Apache attack helicopter that crashed near the coast of Oman overnight was brought down by an Iranian drone, two U.S. officials familiar with the Pentagon’s latest assessment tell ABC News.

The investigation has not yet determined whether Iran intended to use the drone to attack the helicopter, they added.

The drone involved in the incident is the same type Iran typically uses to target ships transiting the Strait of Hormuz, one official said.

-ABC News' Shannon K. Kingston and Luis Martinez

The autonomous surface sea drone used to rescue the two Apache crewmembers picked them up and transported them to another location on the water where they were then hoisted up to a helicopter for further transport, according to a U.S. official.

U.S. Central Command said it disabled a vessel, the M/T Settebello tanker, as it transited in the Gulf of Oman on Wednesday.

Three Indian crewmembers have been reported missing after the attack, according to the Indian Ministry of Foreign Affairs.

A U.S. aircraft fired precision munitions at the ship’s engine crew "after the crew repeatedly failed to comply with directions from American forces," CENTCOM said in a statement.

Twenty-one Indian crewmembers of the Settebello were rescued and India is coordinating with Omani authorities regarding the operation, the Indian ministry said.

Earlier the United Kingdom Maritime Trade Operations Center issued a warning saying local authorities received a report the tanker "experienced a fire in their engine room."

The incident occurred 20 nautical miles northeast of Sohar, Oman, the UKMTO warning said.

India's Foreign Ministry is condemning the attack on the ship saying the targeting of commercial shipping and civilian infrastructure must end.

"We reiterate our call for immediate de-escalation of tensions, and the conclusion of ongoing negotiations for a diplomatic solution so that peace and stability can return to the region," the statement said.

The International Maritime Organization said it "strongly condemns" the attack of the ship sailing under the flag of Palau.

"This is simply unacceptable. My thoughts are with the families of the three missing seafarers and with all those awaiting news of the crew members," IMO Secretary-General Arsenio Dominguez said in a statement.