's deep-sea manned submersible Jiaolong suffered an engine failure in southwestern Indian Ocean on Saturday and was forced to surface ahead of schedule.

Before the sub reached the seafloor, Fu Wentao, who piloted Jiaolong, had received an insulation alert and managed to deal with the trouble.

"However, when we started to collect rocks and sulfide, we heard big noise from the sub. I thought the sub might collide with some seabed rocks, but the noise was still there when I took the sub away," Fu said.

Engineers on the support ship, "Xiangyanghong 09", believed that the noise might be an engine problem and the chief commander of the mission, Yu Hongjun, ordered to surface the sub according to safety regulations.

The malfunction was later confirmed to be the head engine failure, and the sub was pulled back to the support ship at 12:35 p.m. (GMT+4:00), five hours ahead of schedule.

"It's lucky that the sub stopped the mission in time and came up as soon as possible, or it might be in danger," engineers said.

Engineers are replacing the broken engine on Saturday night.

Jiaolong, China's first deep-sea submersible, collected rocks of 9.5 kg, chimney vent of 0.9 kg and sulfide of 0.5 kg in a hydrothermal area.

Reaching its deepest depth of 7,062 meters in the Pacific's Mariana Trench in June 2012, Jiaolong is on a four-month expedition in the southwestern Indian Ocean.