At least 20 migrants died and hundreds more are missing and feared dead after their overcrowded boat capsized in stormy seas off the coast of Libya, an international migration official and Libyan police said Tuesday.

The capsized boat was carrying up to 257 people and a second vessel with about 350 migrants aboard was rescued, Laurence Hart, an official of The International Organization for Migration, told The Associated Press. At least 20 bodies were recovered and more than 20 people were rescued from the overturned boat, Hart and a Libyan police official said.

"The first boat was rescued and is back to Tripoli. All of them are alive and safe," Hart said. "The second boat, I believe 240 people are missing. Rescue was quick for the first boat because they were near an oil platform that notified the Libyan coastal guards who quickly rescued the migrants," he added.

"For the second one, it is believed to be in the same area," Hart said.

Both boats carried migrants from Africa and the Middle East, some of them Syrian Kurds, Hart said.

A Libyan police official said the accident and the rescue of the 20 migrants occurred on Friday. The second boat was rescued on Sunday, he said.

"We only found one body of a migrant hugging a wooden pole," said the police official, who spoke on condition of anonymity because he was not authorized to speak to the media.

Hart said the boat overturned about 30 miles north of Libyan coastal town of Maleta and about 50 miles west of Tripoli.

"This is the typical route for migrants from Libya toward Italy," Hart said.

He said the cause of the accident was not immediately know but high winds and overcrowding were likely factors.

It was not immediately know where the boats sailed from in Libya.

The police official said 17 of those rescued went to hospitals in Libya in poor condition because they had been in the sea for a long time with no food or water to drink. He said the capsized boat had not yet been found.