Thailand's government on Friday signed a 290 billion baht ($9.36 billion) agreement with a consortium of local firms to upgrade and expand U-Tapao airport and its surrounds and turn it into a third international gateway for Bangkok.

It is part of a major government infrastructure project in Thailand's eastern industrial zone and will make U-Tapao a third commercial hub serving the region after Suvarnabhumi Airport and low-cost terminal Don Mueang.

It also covers construction of Airport City adjacent to the airport, a centre for logistics and supporting industries such as aircraft maintenance, repair, aviation training and fuel storage.

The consortium for U-Tapao, BBS Joint Venture, comprises Bangkok Airways, Bangkok Skytrain operator BTS Holdings Pcl and Sino-Thai Engineering and Construction Pcl (STEC), which will hold stakes of 45%, 35% and 20%, respectively.

Construction is expected to take three years and the companies will have 47 years to operate and maintain the airport and aviation city.

The projects together will cover an area of 1,040 hectares (10.4 square km), said STEC President Pakpoom Srichamni.

"The group has a strong partner with Narita International Airport ... (which) will be jointly managing the U-Tapao Airport," Bangkok Airways CEO Puttipong Prasarttong-Osoth said.

The Vietnam-era U-Tapao airport is located about two hours by road from Bangkok.

Last year the government approved a project worth $7 billion for a high-speed rail service linking the three airports.

(Reporting by Panarat Thepgumpanat and Chayut Setboonsarng; editing by Martin Petty and Jason Neely)