HANOI (Reuters) - China sent ships on Friday in response to a request from Vietnam to help find a coastguard plane that crashed with nine personnel aboard while looking for a missing fighter jet and pilot, Vietnam’s defense ministry said.

The CASA turboprop plane went down on Thursday in the Gulf of Tonkin, between Vietnam’s northern coastline and China’s Hainan Island, where the rescue team had found some debris and personal items, the ministry said in a statement.

The Airbus-made plane was searching for a Sukhoi SU-30 MK2 fighter jet and a missing pilot that went off radar on Tuesday. One of the two fighter pilots was rescued from the sea the following day.

Thousands of Vietnamese coastguard, border guard, navy, air force and fishermen have been searching for the aircraft and the second pilot.

China sent one rescue and two coastguard boats to help search for the CASA plane in response to Vietnam’s request for assistance and to allow its vessels to enter the Chinese side of a maritime boundary agreed between the two countries. The coastguard plane went down in bad weather and low visibility.

Vietnam has suffered a series of accidents in the past two years with its aging helicopters, but plane crashes have been rare.

It is currently overseeing its biggest military buildup in four decades and wants to upgrade its air and sea defenses, including plans to purchase fighter jets, a strategy experts say is aimed at building a deterrent against China’s military rise.

The Communist parties that rule China and Vietnam are historically close, but tensions are high over territorial disputes between them in the South China Sea.

Vietnam has stepped up efforts to strengthen its coastguard, with help from Japan, which has its own maritime squabbles with China, and the United States, which has repeatedly locked horns with Beijing and insists it has stake in ensuring freedom of navigation and flight in the South China Sea.