SEOUL, South Korea -- As visiting U.S. President Barack Obama offered South Koreans his condolences Friday for the ferry disaster, the South Korean government conceded that some bodies have been misidentified and announced changes to prevent such mistakes from happening again.

There have been several reports in South Korean media this week of bodies going to the wrong families, with the error sometimes caught only after the remains were taken to a funeral home. An "action plan" released by the government-wide emergency task force acknowledged that "there have been cases where the victims were wrongly transferred."

Remains will be transferred to families when there is a match using DNA testing or fingerprint or dental records, the task force said. The transfer will be temporary when a body is matched though identification or physical description, and authorities will wait for more authoritative evidence before making the transfer permanent.

Divers have recovered 183 bodies so far, but 119 remain missing and are feared dead in the dark rooms of the submerged vessel.

Search officials including a navy spokesman and a diver said 35 of the ferry's 111 rooms have been searched so far, Yonhap news agency reported. They said 48 of the bodies recovered were found were in a single large room built to accommodate 38.

The ferry sank April 16 on its way from Incheon port to the southern tourist island of Jeju. More than 80 per cent of the 302 dead and missing are students from a single high school in Ansan, south of Seoul.

Obama arrived Friday afternoon at the Blue House, South Korea's presidential residence, and presented President Park Geun-hye with an American flag that flew over the White House the day the ship sank. His first South Korean visit since Park took office last year was aimed at issues including North Korea, but he noted that his trip comes at a time of "great sorrow."

"So many were young students with their entire lives ahead of them," Obama said, invoking his two daughters, both close in age to many of the ferry victims. "I can only imagine what the parents are going through at this point, the incredible heartache."

Accepting the flag, Park drew a parallel between the way Americans pulled together after the 9-11 attacks and the resilience of South Koreans following one of the worst maritime disasters in their country's history.

"The Korean people draw great strength from your kindness," she said.

Obama also said he was donating a magnolia tree from the White House lawn to Danwon High School in Ansan in honour of the lives lost and as a symbol of friendship between the U.S. and South Korea.

Eleven crew members, including the captain, have been arrested on suspicion of negligence and abandoning people in need. Prosecutor Yang Jung-jin of the joint investigation team said Friday that the cause of the sinking could be due to excessive veering, improper stowage of cargo, modifications made to the ship and tidal influence. He said investigators will determine the cause by consulting with experts and simulations.

The ferry Sewol was carrying an estimated 3,608 tons of cargo, said Moon Ki-han, a vice-president at Union Transport Co., which loaded its cargo. That's also more than three times what an inspector who examined the vessel during a redesign said it could safely carry. It also far exceeds what the captain claimed in paperwork: 150 cars and 657 tons of other cargo, according to the coast guard.

The Korean Register of Shipping inspector's report said that changes made to the ship meant that it had to carry no more than about 1,000 tons of cargo, while taking on more than 2,000 tons of water as ballast to ensure stability. Before the modifications, the report said, the ship could handle more than 2,500 tons of cargo and needed only about 1,000 tons of water ballast.

Yet the coast guard says shipowner Chonghaejin Marine Co. Ltd. reported cargo capacity of 3,963 tons -- a number unchanged from that reported by the Sewol's previous Japanese owner before the ship was redesigned. It was unclear why the earlier maximum tonnage noted in the register document was lower than that provided by either owner.

A naval architecture expert said Friday that the reported load could have set the ship tipping over with a significant turn. Tracking data show the ship turned 45 degrees before sinking, and crew members have reportedly said that they had tried to make a much less severe turn.

"The ship would suddenly fall even with just a small turn. It should not make a sharp turn," said Lee Kyu Yeul, professor emeritus in ship and offshore plant design at Seoul National University's Department of Naval Architecture and Ocean Engineering. "It should make a huge circle with 1 or 2 degrees of turn, but (the Sewol) made a small circle. So it fell."

Officials with South Korea's maritime ministry and coast guard each said they were not aware of the Sewol's cargo capacity, and that it was the shipping association's job to oversee it. The shipping association is private and is partly funded by the industry it regulates.

An official at the shipping association declined to talk to media by phone, saying it is under investigation by prosecutors.

Prosecutors have raided and seized documents at the Korean Register of Shipping and the Korea Shipping Association, which regulates and oversees departures and arrivals of domestic passenger ships, according to officials at both organizations who spoke on condition of anonymity because they were not authorized to speak about matters under investigation.