(CNN) -- Searchers rescued at least 53 people and about 33 are missing after a ferry capsized in the Pacific Ocean overnight near Tonga's capital, Nuku'alofa, a search and rescue officer said early Friday.
His number of those rescued is three less than the number posted in a news release on the Web site of New Zealand's Ministry of Foreign Affairs and Trade. The ministry said more than 80 passengers and crew were on board.
The vessel, carrying mostly Tonga nationals, sank about 11 p.m. Thursday (6 a.m. ET), said Keith Allen of the New Zealand National Rescue Coordination Centre, which is coordinating the search. The search was centered about 86 kilometers (53 miles) north of Nuku'alofa, the ministry Web site said.
The ferry, the Princess Ashika, was sailing in the Ha'apai group of islands in northern Tonga, the Web site said.
Vessels responding to distress calls were on the scene within two hours, Allen said. Rescuers immediately found survivors in the water and in life rafts. He said 13 life rafts were found, but some of them were empty.
Allen told CNN that search boats had dropped their anchors, fearful they might run over survivors in the dark. They plan to continue the search at daylight, he said.
Authorities don't know what caused the ferry to sink, he said.
Allen said rescuers were awaiting a response from the U.S. Coast Guard, from whom they have requested helicopters.
New Zealand Prime Minister John Key expressed his country's sympathy and concern to Tongan Prime Minister Fred Sevele, according to the ministry Web site. Both men are attending the Pacific Islands Forum Leaders meeting in Cairns, Australia.
"We are deeply concerned about those still missing," New Zealand Foreign Minister Murray McCully said, according to the Web site. He also is in Cairns.
"Two New Zealand Air Force Orions are assisting with the search for survivors, and our authorities are in close touch with Tongan officials in case further support is required," he said.
New Zealand officials also will discuss providing interim ferry services, he added. Tonga is northeast of New Zealand.
"The fact that this disaster comes so close behind the recent sinking in Kiribati in which 33 people lost their lives is doubly tragic for the Pacific region," McCully said.