At least 10 U.S. sailors are missing after the USS John S. McCain collided with a tanker early Monday east of Singapore, the second accident involving a ship from the Navy's 7th Fleet in the Pacific in two months.

The Navy said five other sailors were hurt in the collision between the guided-missile destroyer and the 183-meter (600-foot) Alnic MC, an oil and chemical tanker. Four of them were evacuated by a Singaporean navy helicopter to a hospital Singapore for treatment of non-life threatening injuries and one did not require further medical attention.

The 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.

It wasn't immediately clear if the Liberian-flagged Alnic sustained damage or casualties.

The McCain was damaged on its port side aft, or left rear, from the collision that happened at 5:24 a.m., the Navy's 7th Fleet said, but was heading to port under its own power.

Malaysia's navy chief Ahmad Kamarulzaman Ahmad Badaruddin tweeted that two ships as well as aircraft from its navy and air force have been deployed to help look for the missing U.S. sailors.

The Navy said Osprey aircraft and Seahawk helicopters from the USS America were assisting in the search. It also said tugboats and Singaporean naval and coast guard vessels were in the area to render assistance.

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

The collision is 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 hit each other in waters off Japan.

The Fitzgerald's captain was relieved of 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.

President Donald Trump has expressed concern for the McCain's crew.

Trump returned to Washington on Sunday night from his New Jersey golf club. When reporters shouted questions to him about the McCain, he responded, "That's too bad."

About two hours later, Trump tweeted that "thoughts and prayers" are with the McCain's sailors as search and rescue efforts continue.

The 154-meter (505 foot) McCain, named after U.S. Sen. John McCain's father and grandfather who were both U.S. admirals, is based at the 7th Fleet's homeport of Yokosuka, Japan. It was commissioned in 1994 and has a crew of 23 officers, 24 chief petty officers and 291 enlisted sailors, according the Navy's website.