The construction of the long-awaited high-speed railway connecting Thailand and China started on Thursday.

Premier Li Keqiang sent a letter of congratulation, saying the China-Thailand Railway is a flagship project of Belt and Road Initiative and will improve the level of regional infrastructure construction.

Li said he hoped both sides can maintain close cooperation and finish the construction of the first phase with high quality, and push forward the second phase of the project and strive for an early realization of the interconnection of the China-Thailand Railway and the China-Laos Railway.

Thanin Somboon, Thai director-general of department of highways, said in a news release on Tuesday that the construction of the first 3.5-kilometer section will be a great chance for Thai and Chinese engineers to learn from each other.

On Nov 24, Thai Transport Minister Arkhom Termpittayapaisith said the construction of the remaining 249 km would start gradually after being put out to tender next year, and the full railway is set for completion in 2021.

The first phase of the high-speed project, linking Bangkok and the northeastern province of Nakhon Ratchasima, is set to cost 179 billion baht ($5.4 billion) and would become Thailand's first high-speed railway with a maximum speed of 250 km/h.

Under the deal, the Thai government will pay for the construction by issuing bonds or seeking loans from banks, said Arkhom, adding that China will design it and provide engineers, track systems and equipment.

According to the proposal, the railway is expected to bring about 2 million more Chinese tourists to Thailand every year and will make it easier for Thailand to export its products.

Xu Liping, director of Southeast Asian studies at the Chinese Academy of Social Sciences, said the railway is a pilot project in ASEAN countries and it will stimulate alongside regions comprehensively.

The second phase of the project, which will link up with a China-Laos line that is currently under construction, is allowing travelers to make the trip between Bangkok and the Lao capital of Vientiane in just four hours, and even travel all the way up to Kunming in China's Yunnan province.

The railway will make Thailand become a transportation hub in ASEAN countries, Xu said.

In the future, the railway will be extended southward to Kuala Lumpur, Malaysia and Singapore, as part of the pan-Asia railway network spanning from Kunming through Laos and connecting Thailand, Malaysia and Singapore.

Zhu Xijun, general manager of the Southeast Asia Company of China Railway Construction, said: "After the railway is put into use, people will enjoy a much more convenient transportation between China and Thailand."