South Korean authorities are sending more aircraft and ships to the site of a shipwreck, which has left at least 20 dead and more than 270 missing based on latest tally.

Three large salvage ships are expected to arrive at the scene of the sunken ferry Sewol early Friday morning off Jindo Island, near the southwestern tip of South Korea, the West Sea Coast Guard told reporters at a briefing in Jindo on Thursday.

South Korean coast guard and navy authorities have dispatched a total of 29 aircraft and 171 vessels for rescue, it said.

Military and civilian divers tried to enter the ferry for dozens of times on Thursday but failed finally. The rising winds, waves and rains held back the rescue effort including the underwater operation.

The families of the ferry's passeners were allowed to embark on the rescue ships to visit the shipwreck site, and many of them burst ino tears as they saw the tip of the wreckage.

It took more than two hours for Xinhua reporters aboard a rescue ship to reach the site from the nearest port, but the ship had to refurn after a brief stay due to the harsh weather conditions over there.

A local rescue expert told Xinhua that the South Korean coast guard and navy are prepared for underwater mission, but at a depth of 45 meters with high winds and waves on the surface, the conditions are very dangerous for the divers to operate.

Many civilian divers are also joining the mission and their best hope is a quick imporvement in weather conditions, he added.

The passenger ship with 475 people aboard, mostly high school students, capsized in waters off South Korea's southwest coast Wednesday morning. Local broadcaster YTN's latest reports said that at least 20 people were dead and more than 270 others still missing.

It was reported that the ferry was believed to have veered off course as it departed some two and a half hours later than scheduled due to a heavy fog.

The coast guard said they are still analyzing the track of the ship's route supplied by marine company to determine whether Sewol is far off its intended route.

The 6,825-ton passenger ship made an abrupt turn at around 8:48 a.m. local time Wednesday, according to the automatic identification system (AIS) data offered by the South Korean Oceans and Fisheries Ministry.

Why the captain suddenly made the turn had yet to be known, but the change in direction was believed to have moved some 180 cars and trucks and more than 1,100 tons of shipping containers on the deck of the ship to one side, driving the ship to lean to the port side gradually.

The Sewol reportedly tilted first, rolled over on the port side and capsized before being totally submerged in the waters at around 11:30 a.m. local time Wednesday. It took around two and a half hours until the ship sank.

The passenger ship was originally believed that it may have run aground as some rescued passengers said they heard a thumping sound on the bow before the ship sinks.

The thud was estimated to be a noise made when the vehicles and containers were out of their place and crashing each other.

The coast guard said experts from South Korea's National Scientific Research Institute, National Marine Research Institute, Marine Science and Technology Institute and other agencies are investigating whether the ferry was overloaded or has some structural or systematic defects.

The ship's captain and crew members are also questioned by the police to find out what went wrong.

The West Sea Coast Guard also said the possibilities of the ferry hit an underwater rock remains low as the underwater image shows that there are few submerged rocks in the waters.

The final results of the investigation will be published after analyzing concrete evidences, officials said.

On Thursday, Chinese President Xi Jinping and Premier Li Keqiang extended condolences to their respective South Korean counterparts over a ferry accident, which has left a significant number of people dead or missing.

"I have learned with profound sorrow of the severe accident of your country's passenger ship 'Sewol', which has left a significant number of people dead or missing, including many young students in particular, " Xi said in a message to South Korean President Park Geun-hye.

Xi, on behalf of the Chinese government and people and in his own name, mourned the dead and sent condolences to those injured and to the bereaved families.

He said that he pays great attention to this accident and that China is willing to provide support and assistance to South Korea.

"The Chinese people and the South Korean people enjoy profound friendship," Xi said.

Premier Li also sent a message of condolences on Thursday to his South Korean counterpart, Chung Hong-won, over the tragedy.

"Sewol" carrying 475 people, mostly high-school students, capsized off South Korea's southwest coast Wednesday, leaving at least nine people dead and 287 others missing.

Singapore Prime Minister Lee Hsien Loong also conveyed his condolences on Thursday to Park over shipwreck, saying he was very saddened to learn of the incident.