* Aircraft sank after crashing into sea, wreckage found

* Ships and helicopter pressed into the search

* Plane lost contact 13 minutes after takeoff from Jakarta

* Survivors are not expected

JAKARTA - Indonesian President Joko Widodo says he has ordered the National Commission for Transportation Safety to investigate crash of a Lion Air plane.

He said rescuers are making their best efforts to find victims and urged Indonesians "to keep on praying." Widodo, speaking in Bali where he was attending a conference, said he feels the anxiety of families and hopes they can remain calm while rescuers are working hard at the crash location at sea northeast of Jakarta.

The national search and rescue agency said human remains have been recovered from the crash area. Its director of operations, Bambang Suryo Aji, told a news conference the search effort is focusing on finding bodies, and survivors are not expected.

The aircraft with 189 people on board crashed into the sea and sank on Monday soon after taking off from the capital, Jakarta, on a flight to a tin-mining region, officials said.

Lion Air flight JT610, an almost new Boeing 737 MAX 8, lost contact with ground officials 13 minutes after takeoff, and crashed about 15 km (9 miles) off the coast.

Indonesia is one of the world's fastest-growing aviation markets, although its safety record is patchy. If all aboard have died, the crash would be the country's second-worst air disaster since 1997, industry experts said.

"We don't know yet whether there are any survivors," search and rescue agency head Muhmmad Syaugi told a news conference, adding that no distress signal had been received from the aircraft's emergency transmitter.

"We hope, we pray, but we cannot confirm."

Items such as handphones and life vests were found in waters about 30 metres to 35 metres (98 to 115 ft) deep near where the plane, a Boeing 737 MAX 8, lost contact, he said.

"We are there already, our vessels, our helicopter is hovering above the waters, to assist," Syaugi said. "We are trying to dive down to find the wreck."

Ambulances were lined up at Karawang, on the coast east of Jakarta and police were preparing rubber dinghies, a Reuters reporter said. Fishing boats were being used to help search.

At least 23 government officials were on board the plane, which an air navigation spokesman said had sought to turn back just before losing contact.

Edward Sirait, chief executive of Lion Air Group, told reporters the aircraft had a technical problem on a flight from the resort island of Bali to Jakarta but it had been "resolved according to procedure".

Sirait declined to specify the nature of the issue but said none of its other aircraft of that model had the same problem. Lion had operated 11 Boeing 737 Max 8s and it had no plan to ground the rest of them, he said.

The accident is the first to be reported involving the widely sold Boeing 737 MAX, an updated, more fuel-efficient version of the manufacturer's workhorse single-aisle jet.

Privately owned Lion Air said the aircraft had been in operation since August, was airworthy, with its pilot and co-pilot together having accumulated 11,000 hours of flying time.

The head of Indonesia's transport safety committee said he could not confirm the cause of the crash, which would have to wait until the recovery of the plane's black boxes, as the cockpit voice recorder and data flight recorder are known.

"The plane is so modern, it transmits data from the plane, and that we will review too. But the most important is the blackbox," said Soerjanto Tjahjono.

Safety experts say nearly all accidents are caused by a combination of factors and only rarely have a single identifiable cause.

The weather was clear, Tjahjono said.