Manila - The Philippine coastguard rescued 21 people from three cargo and fishing vessels that sank in rough waters as Typhoon Hagupit gathered strength off the country's northeast coast, officials said on Monday.

Disaster officials also braced for possible landslides and flooding after a category 3 warning signal was raised across six northern provinces on the main island of Luzon, meaning a tropical cyclone was imminent.

Schools in the affected areas were closed and troops and police officers put on standby. Some of the schools will be used as temporary shelters for displaced families if necessary.

Hagupit, which means "lashing" in Filipino, was about 150 km (90 miles) east-northeast of the Philippines at around 0800 GMT, and was expected to bring heavy rain. It was moving west-northwest at about 20 km (12 miles) per hour with gusts of up to 210 km (130 miles) per hour.

In Taiwan, authorities issued land and sea warnings on Monday as the fifth typhoon of the season headed towards its south coast.

Hagupit was expected to brush Taiwan and parts of the northern Philippines overnight, although rain and rough seas were reported as far as the central Philippines on Monday. The storm was expected to reach southern China, including Hong Kong, by Wednesday.

The Philippine coastguard said 13 of those rescued were from a wooden-hulled cargo vessel that sank off a central Philippine island, while another eight were picked up off northern Luzon from a capsized fishing boat.

"We're still looking for another fisherman in waters off La Union province in the north," Armand Balilo, a coastguard spokesman, said, adding a third cargo vessel sank near Sibuyan island in the central Philippines.

The coastguard did not give details about the crew and passengers of the third ship.

"We've alerted local disaster agencies to evacuate residents in landslide-prone areas as well as those in coastal villages to avoid loss of lives," the weather bureau said in an advisory on Monday.

In June, about 600 people were killed and about 15 billion pesos ($325 million) worth of property and farms were destroyed by a typhoon that hit the central Philippines. Seven cargo vessels and a ferry carrying 800 people also sank due to waves and strong winds.

Tropical storms in the region gather intensity from the warm ocean waters and frequently develop into typhoons that hit Taiwan, Japan, the Philippines and southern China during a season that lasts from early summer to late autumn.

($1 = 46.15 pesos) (Reporting by Manny Mogato in Manila and by Ralph Jennings in Taiwan; Editing by Raju Gopalakrishnan and Alex Richardson)