Typhoon Tembin did not make landfall in Vietnam and was downgraded to a weakening tropical depression as it headed for the Gulf of Thailand, local authorities say.

The storm had been forecast to wreak havoc on the country's southern provinces on Tuesday morning, the National Steering Committee for Natural Disaster Prevention and Control said.

It approached the coastline of Vietnam's southernmost provinces Ca Mau and Bac Lieu overnight as winds slowed down to 60km/h, compared to 135km/h on Monday morning, authorities said.

Out of 1.2 million people ordered to leave ahead of the storm's arrival, authorities had already evacuated 430,000 people by Monday. A further 346,000 fishermen on 70,000 fishing boats were ordered to remain anchored at ports.

Despite the storm's downgrading, authorities warned it was still likely to cause heavy rain of up to 181 millimetres in southern provinces.

Initial forecasts had the storm packing as much power as Typhoon Linda, which left more than 3000 people dead or missing when it struck the region 20 years ago.

Tembin blew out of the Philippines on Sunday, leaving over 200 people dead and emergency services searching for dozens of missing people due to flash floods and landslides in the country's south.

Australian Associated Press - December 25, 2017