JAKARTA, Indonesia -- The first full day of a growing, multi-nation hunt for an AirAsia passenger jet lost over Indonesian waters ended earlier today with few clues to its disappearance and a grim acknowledgment by officials that "the worst may have happened."

Helicopters and surveillance aircraft from Indonesia, Malaysia, Singapore and Australia returned to their bases, some after flying 10 hours or more over a choppy Java Sea, the focal point of the search for the airliner that was carrying 162 mostly Indonesian passengers.

The first tantalizing possible traces of the missing Airbus A320-200 - which lost contact with air-traffic control during a two-hour flight to Singapore from the Indonesian city of Surabaya on Sunday morning - were inconclusive or ultimately discounted.

Air Force spokesman Hadi Tjahjanto told LA The Times that searchers in a helicopter and C-130 plane had seen an oil slick about 105 nautical miles off Indonesia's Belitung island near the Karimata Strait, which connects the archipelago nation to Singapore.

"We're checking whether it's jet fuel or fuel from a ship," he said.

An Australian Orion aircraft spotted "suspicious objects" in the sea near Nangka island, northeast of the Belitung, about 700 miles from where the plane lost contact, prompting teams to rush toward the area under cloudy skies.

But Bambang Soelistyo, the head of Indonesia's National Search and Rescue Agency, which was leading the effort, said the weak signal detected by the Australian plane came from a personal locator beacon, not the missing jet's emergency transmitter.

Bambang said that based on the coordinates of Indonesia AirAsia Flight 8501's last known location, investigators believed it had crashed into the Java Sea north of Jakarta.

"My goal is to locate it as soon as possible," Bambang told a news conference in Jakarta. "We're doing the best we can."

Indonesian Vice President Jusuf Kalla said "bad weather" was hampering the search but expressed hope that the plane would be found.

"We don't set deadlines. What is important is we find the plane and its passengers," Kalla said.

Asked about the prospects of finding survivors more than 36 hours after the flight went missing, Kalla said: "We pray for them but we realize the worst may have happened."

Bambang said teams searched an area comprising roughly 66,000 square miles in four sectors on Sunday, concentrating on a 250-mile-wide stretch of the Java Sea between the Indonesian islands of Sumatra and Borneo. On Monday the search expanded to the north to include the Karimata strait and the coasts of Belitung island and West Kalimantan province, he said.

Indonesia's armed forces had deployed transport helicopters and naval ships while Malaysia and Singapore had each sent C-130 Hercules transport aircraft and three ships to help ferry teams to and from the search site. Early Monday, Australia's Orion surveillance aircraft joined the effort.

If wreckage isn't found at the water's surface, investigators likely would begin scouring the sea floor for the Airbus A330-200 -- probably requiring contributions of advanced ships and equipment from other nations.

"If that's the case, we'll have difficulty determining the location because our equipment is not adequate," Bambang said.

Singapore civil aviation officials said they were preparing to send two teams of specialists andunderwater locator beacons to help find the missing jet's flight data recorders.

China offered to send airlines and ships to join the search and rescue efforts, foreign ministry spokeswoman Hua Chunying said in Beijing. South Korea said it would send a surveillance plane later this week, the Yonhap news agency reported.

In Washington, Pentagon officials said Sunday they were ready to assist but had not been asked.

AirAsia executive chief Tony Fernandes defended his airline's safety record, saying it had carried 220 million passengers in 13 years and never had a fatal accident.

"Until we have a full investigation, we don't want to speculate," Fernandes told a news conference in Surabaya. "It's premature to talk (about what went wrong) at the moment. We are confident in our ability to fly people. We'll continue to be strong and continue to carry people who never fly before."

Indonesian transportation ministry Ignasius Jonan said the government would review AirAsia's operations "to ensure that in the future its activity will be better." The low-cost carrier, which is based in Malaysia and operates mainly short flights across Southeast Asia, has a strong safety record and is widely regarded as one of the world's most successful budget airlines.