Media playback is unsupported on your device Media caption Asylum seekers try to reach Australia by sea every year

A rescue operation is under way after a boat carrying hundreds of asylum seekers towards Australia's Christmas Island sank off the Indonesian coast.

Strong waves wrecked the fibreglass boat about 90km (55 miles) out to sea east of Java, rescue officials said.

At least 250 people - believed to be from Afghanistan, Turkey and Iran - were said to be on board, but some reports put the number nearer to 400.

So far 33 people have been saved - by fishermen on Saturday evening.

They included two children, aged 8 and 10, found clinging to wreckage, AP news agency reports.

Survivors said it was more than five hours before they were spotted by the fishermen.

"It's really a miracle they made it," said Kelik Enggar Purwanto, a member of the national search and rescue team, told AP.

Image caption The survivors said they clung to wreckage for more than five hours before being found by fishermen

'Life jacket fight'

The Indonesian navy has sent two helicopters to sweep the eastern coastline of the island of Java in an attempt to find any more survivors.

But bad weather and reduced visibility is hampering their efforts, officials say.

Police told state-run news agency Antara that the vessel appeared to have been carrying more than twice its capacity.

Survivor Esmat Adine, a 24-year-old Afghan migrant, told the agency that the ship had started rocking violently from side to side, triggering panic among the tightly packed passengers.

"That made the boat even more unstable and eventually it sank," he said.

Another survivor, 17-year-old Afghan student Armaghan Haidar, told the AFP news agency he was among about 100 people who managed to escape and cling to the wreckage, but he estimated another 100 passengers had been trapped inside. Between 20 and 30 people had had life jackets, he said.

According to Afghani Khadzim Huzen, a fight broke out for the life jackets as there were only 25 on board, nine of them allocated to the crew, AP reports.

'Terrible tragedy'

Thousands of asylum seekers try to reach Australia by sea every year, many paying huge sums of money to people-smugglers in Indonesia to transport them.

The BBC's Karishma Vaswani in Jakarta says Indonesia is often the transit point for refugees as international borders in the region are hard to control.

Australian Home Affairs Minister Jason Clare described the sinking as "a terrible tragedy".

Australia's policy of dealing with illegal migrants has been the subject of heated political debate in the country.

Opponents of Prime Minister Julia Gillard say her lack of action has emboldened the people smugglers to try to send as many to Australia as possible.

Australia's asylum policy has been in crisis since a court ruled a plan to swap refugees with Malaysia unlawful.