Vessels and aircraft from Singapore, Indonesia, Malaysia and the United States are scouring Southeast Asian waters for 10 missing American sailors after an early morning collision Monday between the USS John S. McCain and an oil tanker ripped a gaping hole in the destroyer’s hull.

The collision east of Singapore between the guided missile destroyer and the 183-meter (600-foot) Alnic MC was the second involving a ship from the U.S. Navy’s 7th Fleet in the Pacific in two months.

Four other sailors were evacuated by a Singaporean navy helicopter to a hospital in the city-state for treatment of non-life threatening injuries, the Navy said. A fifth injured sailor did not require further medical attention.

The USS John McCain had been heading to Singapore on a routine port visit after conducting a sensitive freedom-of-navigation operation last week by sailing near one of China’s man-made islands in the South China Sea.

The Navy’s 7th Fleet said “significant damage” to the McCain’s hull resulted in the flooding of adjacent compartments including crew berths, machinery and communications rooms. A damage control response prevented further flooding, it said.

The destroyer was damaged on its port side aft, or left rear, in the 5:24 a.m. collision about 4.5 nautical miles (8.3 kilometers) from Malaysia’s coast but sailed on to Singapore’s naval base under its own power. Malaysia’s Maritime Enforcement Agency said the area is at the start of a designated sea lane for ships sailing into the Singapore Strait, one of the world’s busiest shipping lanes.

A photo tweeted by Malaysian navy chief Ahmad Kamarulzaman Ahmad Badaruddin showed a large rupture in the McCain’s side near the waterline. Janes, a defense industry publication, estimated the hull breach was 3 meters (10 feet) wide.

One of the injured sailors, Operations Specialist 2nd Class Navin Ramdhun, posted a Facebook message telling family and friends he was OK and awaiting surgery for an arm injury.

He told The Associated Press in a message that he couldn’t say what happened. “I was actually sleeping at that time. Not entirely sure.”

The Singapore government said no crew were injured on the Liberian-flagged Alnic, which sustained damage to a compartment at the front of the ship some 7 meters (23 feet) above its waterline. There were no reports of a chemical or oil spill.

Several safety violations were recorded for the tanker at its last port inspection in July.

Singapore sent tugboats and naval and coast guard vessels to search for the missing sailors and Indonesia said it sent two warships. Malaysia said three ships and five boats as well as aircraft from its navy and air force were helping with the search, and the USS America deployed Osprey aircraft and Seahawk helicopters.

There was no immediate explanation for the collision, and the Navy said an investigation would be conducted. Singapore, at the southernmost tip of the Malay Peninsula, is one of the world’s busiest ports and a U.S. ally, with its naval base regularly visited by American warships.

The collision was the second involving a ship from the Navy’s 7th Fleet in the Pacific in two months. Seven sailors died in June when the USS Fitzgerald and a container ship collided in waters off Japan.

The Fitzgerald’s captain was relieved of his command and other sailors were being punished after the Navy found poor seamanship and flaws in keeping watch contributed to the collision, the Navy announced last week. An investigation into how and why the Fitzgerald collided with the other ship was not finished, but enough details were known to take those actions, the Navy said.

The Greek owner of the tanker, Stealth Maritime Corp. S.A., replaced its website with a notice that says it is cooperating with the Maritime Port Authority of Singapore’s investigation and with “other responding agencies.” It says “thoughts and prayers are with the families of the missing U.S. Navy sailors.”

An official database for ports in Asia shows the Alnic was last inspected in the Chinese port of Dongying on July 29 and had one document deficiency, one fire safety deficiency and two safety of navigation problems.

The database doesn’t go into details and the problems were apparently not serious enough for the Liberian-flagged vessel to be detained by the port authority.

U.S. President Donald Trump expressed concern for the missing McCain’s crew.