China and Turkmenistan have agreed to promote bilateral energy cooperation.

Chinese President Xi Jinping, alongside his Turkmen counterpart Gurbanguly Berdymukhamedov, told reporters Tuesday that the two countries "will be long-term, stable and reliable energy partners," China Daily newspaper reports.

Turkmenistan was the first stop of Xi's four-nation Central Asian tour -- his first to the region since taking office in March -- that includes Kazakhstan, Uzbekistan and Kyrgyzstan.

"Today is an important day for Turkmen-Sino relations," Berdymukhamedov said, noting that the two countries signed a joint declaration "to raise the ties to a new level of strategic partnership."

"That means from today on, no matter how the regional and global situation changes, developing the Turkmen-Sino relationship will always be the priority of both nations," Berdymukhamedov said.

In the declaration, the two countries agreed to take measures to guarantee a safe operation of the already-completed Line A and Line B on the Turkmenistan-China pipeline, the world's longest natural gas pipeline. They also said they would work together to complete the construction of Line C and Line D.

It also said they would set up a mechanism to ensure security of the natural gas projects.

Chinese Ambassador to Turkmenistan Xiao Qinghua told Chinese state television broadcaster CCTV that annual gas supply from Turkmenistan to China will reach 65 billion cubic meters by 2020.

On Wednesday, the Chinese and Turkmen presidents jointly announced the completion of the first-phase construction of the Galkynysh gas field located in the south of Turkmenistan, Chinese state-run news agency Xinhua reported.

The Galkynysh, with proven reserves of between four and six trillion cubic meters of natural gas, is the world's second largest gas field.

Experts note that such energy cooperation helps the involved economies to achieve their objectives.

Shi Ze, a researcher on Central Asian studies at the China Institute of International Studies said Central Asia's resource-based economies have high expectations for China as an export market.

"They are also in need of Chinese investment to upgrade their economies," Shi told China Daily.

In addition to its potential for economic cooperation, Central Asia's location also offers the region a key role in geopolitics, Shi added.

China is Turkmenistan's largest trading partner. Chinese Ministry of Commerce figures show that trade volume between China and Turkmenistan in 2012 was $10.37 billion, almost double the figure in 2011.

The Chinese president, in a written interview ahead of his trip published Wednesday by Xinhua, referred to China as "a major energy consumer" and Turkmenistan as "a major energy producer," and said the two countries "enjoy a natural advantage and huge potential in energy cooperation."