The Boeing 777-200 disappeared Saturday on a flight from Kuala Lumpur, the capital of Malaysia, to Beijing with 239 people on aboard. At least 154 of the 227 passengers are Chinese nationals and 38 were from Malaysia, CNN reported.
The Malaysian government conceded Saturday that the passengers, including five children, and the crew are most likely dead.
"I'd just like to say our thoughts and prayers are with the bereaved families," Malaysian Prime Minister Najib Razak told reporters.
The plane's last known position before vanishing was over the South China Sea about midway between Malaysia and Vietnam as it flew at an altitude of 35,000 feet. Xinhua, China's official news agency, reported that planes and helicopters from China, Malaysia, Singapore, the Philippines and Vietnam are involved in the search.
The oil on the surface of the water was first noticed by a search plane from Vietnam and reported by that country's government news agency.
Earlier, Malaysian Airlines officials told CNN air traffic controllers had lost track of Flight MH370 and had "no idea where this aircraft is right now."
The families of Chinese passengers were in a conference area at a Beijing hotel, CNN said.
"My son was only 40 years old," one distraught woman was heard crying as officials led her into the waiting area. "My son, my son. What am I going to do?"
Three of the passengers were reported to be from the United States. The nationalities of some were unclear because at least two people were traveling on passports that had been reported stolen, CNN said.