Saudi Aramco will sign Tuesday memoranda of understanding with U.S. liquefied natural gas producer NextDecade and utility firm Sempra, Aramco's chief executive said, as the oil giant expands in the LNG market.

"The U.S. today, in terms of gas, is almost 100 billion (dollars) in sales ... and it is continuously increasing," Aramco's CEO Amin Nasser told the U.S.-Saudi Investment Forum in Riyadh.

"The U.S. is really a good place to put our investment," he added, noting that under the MoUs Sempra and NextDecade would supply around 6.2 million tons of LNG to Aramco.

The United States is already the world's largest exporter of LNG and producers have plans in place that would double capacity in coming years.

NextDecade last month signed a deal with a subsidiary of Aramco, which is seeking to become a big player in the LNG market, under which the U.S. firm will supply the superchilled gas from its Rio Grande facility for 20 years.

"We do have other investments. So we're looking at, by 2030, almost seven and a half million tons of LNG," Nasser noted, speaking of expansion plans.

Nasser also said that one of the investments that Aramco plans to sign on Tuesday involved an expansion of the Motiva Port Arthur's refinery in the U.S., noting the oil giant would invest $3.4 billion in the refinery.