SEOUL, South Korea — Dozens of rescue boats and helicopters are scrambling to save more than 470 people, including many high school students, caught on a ferry sinking off South Korea's southern coast, officials said.

The ferry with 476 people, including 325 high school students, was sailing to the southern island of Jeju when it sent a distress call Wednesday morning as it began leaning to one side, according to Ministry of Security and Public Administration.

Park Hye-rang, a local coast guard officer, said by phone that 147 passengers had been rescued so far, but gave no further details.

Local media photographs showed the ship heavily tilted onto its side, partially submerged, as helicopters flew overhead and rescue vessels and a small boat covered with an orange tarp floated nearby.

The students are from a high school in Ansan city near Seoul and they were on their way to the Jeju island for a four-day trip, according to a relief team set up by Gyeonggi Province, which governs the city. The ship left Incheon port, just west of Seoul, on Tuesday evening, according to the state-run Busan Regional Maritime Affairs & Port Administration.

A total of 18 helicopters and 34 rescue boats have been sent to the area, Vice Minister Lee Gyeong-og told a televised news conference. He said President Park Geun-hye has ordered a through rescue operation to prevent any human casualties.