A boat packed with around 250 migrants sank off Indonesia's Java island Saturday, search and rescue officials said, with efforts to reach survivors hampered by bad weather and heavy seas.

"A boat carrying around 250 people has sunk south of Prigi beach in eastern Java and we have started a search and rescue effort," the national search and rescue team said in an sms message.

State-run news agency Antara quoted search team member Brian Gauthier as saying: "The boat sank Saturday evening."

"It is somewhat difficult to go on with the search because extreme weather has caused reduced visibility," he added.

Thirty-three people have been rescued and are receiving assistance in the town of Prigi, around 30 kilometres (18 miles) from where the boat sank, Gauthier said, adding that the rescue team believed some passengers were still alive and were likely suffering "severe dehydration".

"They must be evacuated as soon as possible. They can't stay for long in the middle of the sea," he said.

The boat is believed to be a traditional fishing vessel with a capacity of around 100.

A survivor from Afghanistan, 24-year-old Esmat Adine, gave rescuers an estimate for how many passengers were on the boat.

"He did not know exactly how many passengers there were, but he said that four buses with around 60 or more adult passengers each had turned up to the port where they set off," a translator for Adine said.

Adine said the boat had been heading towards Australia's Christmas island.

"After interviewing the passengers, we've learnt that they originate from Afghanistan, Iran and Pakistan. There also are some from Dubai," Watulimo sub-district police chief Muhammed Khoiril told Detik.com on Sunday.

Thousands of asylum seekers head through Southeast Asian countries on their way to Australia every year and many link up with people smugglers in Indonesia for the dangerous sea voyage.

Canberra has failed in its efforts to set up a regional processing centre in neighbouring countries in an attempt to reduce the flow of asylum seekers heading to Australia.