More than 13,000 vehicles remained in line for ferry transportation from Hainan province on Thursday evening due to heavy fog that continued to affect normal ferry services at Haikou ports, local authorities said.

Qiongzhou Strait, which links Guangdong province with Hainan, has been shrouded by a heavy fog for seven days, the longest period since 1951, according to the Hainan provincial meteorological department.

Official data show that by Thursday at 11:30 am, 9,825 vehicles had been ferried in the morning. And by 6 pm, there were still 13,251 vehicles lined up for transportation, with many of them having waited more than 14 hours.

From Feb 15 to 21, ferries carried 86,109 vehicles leaving Hainan, but stranded cars continued to crowd roads near the three ports in Haikou, the provincial capital, as intervals of heavy fog affected normal ferry service, said a statement from the city government on Thursday.

To help ease the traffic jam, the Haikou city government gave government employees a day off on Thursday. The government also asked its civil servants to join local volunteers in providing essential services to stranded passengers.

Hainan, a popular destination for Chinese tourists during the Lunar New Year holiday, received 5.67 million tourists and more than 92,000 cars during the week, statistics from the provincial tourism development commission show.

To compound problems, about 12:46 am on Thursday, a local Hainan freighter Xinying F069 sank after colliding with the passenger ferry Haikou 9 off New Hai'an Port in Xuwen, Guangdong province, with five people from Xinying F069 falling into the water.

Staff members from Haikou 9, which carried more than 970 tourists, immediately conducted rescue work, saving three people. The other two remained missing. The sunken cargo ship has been raised, according to authorities.

Haikou 9 docked safely at Xuwen at 6 am, with all 970 passengers and 140 vehicles. An investigation is underway to determine the causes of the collision, Haikou city government officials said.

The number of cars coming to Hainan from mainland cities and regions has increased by 15 percent a year since Hainan was designated in 2010 by the central government to develop as an international tourism destination. There have been traffic jams from time to time during holidays, but the current one is the most serious.