One of Vietnam's marine surveillance ships participating in rescue effort of cargo ship named Hai Thanh 26. Photo by Vietnam MRCC

Nine Vietnamese men are missing after their cargo ship collided with an unknown vessel and sank in the waters off the country's southern coast early on Tuesday morning, search and rescue forces have reported.

The Hai Thanh 26, with 11 crew members on board, sank about 44 nautical miles to the east of Vung Tau City in Ba Ria-Vung Tau Province while carrying clinker from the northern city of Hai Phong to the Mekong Delta city of Can Tho. Two of the sailors have been rescued, according to the Vietnam Maritime Search and Rescue Center (MRCC). Clinker is used to produce cement.

Rescued crew member Hoang Tien Khoi told the center that his ship sank at around midnight on Monday after the collision, Tuoi Tre reported.

The MRCC has deployed two surveillance ships and asked the Petrolimex 14 oil tanker, which is operating near the scene, to assist the rescue effort.

At around 7 a.m., the Petrolimex 14 found a raft carrying two members of the sunken ship, including Khoi.

Luong Truong Phi, deputy director of MRCC No.3 in charge of the Vung Tau area, told VnExpress the center is coordinating with other ships operating in the area to search for the nine missing crew members.

"We haven't been able to identify the ship that collided and sunk the Hai Thanh 26 yet," said Phi.