(CNN) -- Eighteen people were rescued -- but as many as 21 others were missing -- after a boat sank late Sunday in remote seas off Australia's Cocos Islands, according to Australian Customs and Border Protection Service officials.
A fishing vessel and a commercial ship responded to an alert by the Australian Maritime Safety Authority after the boat sank about 11:15 p.m. Sunday (1515 GMT), a spokeswoman for the customs and border agency said. The sinking occurred about 350 nautical miles northwest of the Cocos Islands, an Australian territory in the Indian Ocean off Australia's west coast.
By early Monday, the customs and border agency had received reports from commercial vessels in the area that at least 18 people had been rescued. The agency said it was believed that 39 people had been on the boat when it sank.
Details about the boat, its port of origin, destination, and the people aboard were not released by Australian authorities.
The Royal Australian Air Force sent a surveillance aircraft to the area and a customs and border agency aircraft was dispatched to aid in a search-and-rescue mission, the spokeswoman said.