A ferry with 251 people on board capsized off the Philippines on Thursday with reports of an unspecified number of "casualties", the coastguard said.

The accident occurred off the town of Real about 70 kilometres (43 miles) east of Manila as the vessel sailed to the island of Polillo in rough weather, coastguard spokesman Armand Balilo told a news conference.

"We have heard (there were) casualties, but we're still validating," he said, adding that rescue helicopters and sea vessels were heading to the site.