The Jiaolong, China's manned deep-sea submersible, on Friday made its third dive into the Mariana Trench, the deepest part of the world's oceans.

"The mission of the sea dive test is to find problems and solve them before the submersible is to put into practical use," said Liu Xincheng, the on-scene party chief.

The vessel set a new national record by reaching the depth of 6,965 meters under the sea in the second dive on Tuesday, surpassing the previous 6,671-meter record in the first dive last Friday.

"The adjustable ballast system was unable to drain away water normally in the second dive, which hindered the submersible from cruising, working on fixed-point and hovering," Liu Feng, the on-scene commander, told Xinhua.

"We will recheck the adjustable ballast system of the vessel and continue to verify its more than 200 indices and functions," he said.

The dive, started at 7 am local time (2100 GMT Thursday), is expected to reach as deep as 6,960 meters.

The submersible plans to make six dives, aiming to reach the depth of 7,000 meters.

The Xiangyanghong 09, the vessel's oceanographic mother ship, arrived at the designated dive zone in the Mariana Trench, located in the western Pacific Ocean, on June 11.