A government source said it was possible the mainland-registered vessel, Zhong Xing 2, sank with the 11 missing crew members trapped aboard, or unable to escape in time.

The source said it was possible poor visibility caused by heavy rain overnight was partly to blame for the collision.

An air and sea search-and-rescue operation co-ordinated by the Guangdong Maritime Rescue Co-ordination Centre was launched, the Hong Kong Marine Department said. Mainland authorities also deployed a fixed-wing plane to search the area.

From Hong Kong, seven Marine Police launches, three fireboats, two diving vessels and a Government Flying Service helicopter were also involved in the search.

Captain Bruce Wong Ho-man, deputy manager of flight operations with the Government Flying Service, said a helicopter was sent out on two sorties – once in the morning and once in the afternoon.

“An Oil slick was spotted together with debris and some floating rings at the scene but no sign of the missing crews or sunken vessel were found,” he said.

The 97-metre Zhong Xing 2 was carrying cement from Hebei province to the port of Haikou in Hainan province at the time the collision occurred, the Marine Department said. It had a crew of 12.

A Marine Department spokeswoman said the mainland-registered vessel collided with a 300-metre Marshall Islands-registered container ship, MOL Motivator, which was leaving for Yantian in Guangdong from Hong Kong.

“Zhong Xing 2 reportedly sank after the collision,” she said. “One of the crew was picked up by a fishing boat, while the other 11 are missing.”

She said the department has alerted other vessels to look out for the missing crews but to stay out of the area.

At 4.30pm, the spokeswoman said the search was still continuing.

She said investigation into the cause of the collision was being carried out by mainland authorities as the incident happened outside Hong Kong waters.