The former Soviet republic, which holds the world's fourth-largest gas reserves, has exported nearly all its production to China since 2009, when the Central Asia-China pipeline opened.

In the middle of the desert, former president Gurbanguly Berdymukhamedov -- who effectively runs the country alongside his son, President Serdar Berdymukhamedov -- formally inaugurated the launch of the fourth of seven planned development phases at Galkynysh.

The ceremony was attended by Chinese Vice Premier Ding Xuexiang, an AFP correspondent saw.

"Turkmen gas is a symbol of happiness -- it is present in every Chinese household," Ding said.

The event featured songs and dances celebrating Turkmen-Chinese friendship, staged with the lavish pomp typical of Turkmenistan's state-sponsored events.

Gurbanguly Berdymukhamedov, officially titled "Hero-Protector" and vested with sweeping powers, presided over the gathering.

Galkynysh, in the Karakum desert about 400 kilometres (250 miles) east of the capital Ashgabat, has been producing gas since 2013 and is the world's second-largest gas field, according to the British consulting firm GaffneyCline.

Expansion works are being carried out by the state-owned China National Petroleum Corporation (CNPC).

On a visit to Ashgabat the day before the ceremony, CNPC chairman Dai Houliang said "the friendship between China and Turkmenistan is as deep as the roots of a tree."