Daniel Russel, the most senior US diplomat to visit Thailand since the military seized power last May, told the generals the US would not "paper over" problems in the country that has traditionally being one of Washington's strongest allies in Asia.

"Thailand is losing credibility in the eyes of its international friends and partners by not moving more quickly to end martial law, to restore civil rights and to ensure that this effort to engineer a new constitution and hold elections is not purely a top-down affair," Mr Russel said.

Mr Russel, the US's Assistant Secretary of State for East Asia and Pacific Affairs, also called for greater judicial fairness in the country only days after a criminal case was filed against former prime minister Yingluck Shinawatra and she was banned from politics for five years by a military-stacked parliament.

"When an elected leader is deposed from office, then impeached by the authority that implemented the coup – and is being targeted with criminal charges while basic democratic processes and institutions are disrupted, the international community is left with the impression that these steps, could, in fact, be politically driven," he said.

Coup leader and now prime minister Prayuth Chan-ocha played down Mr Russel's comments, pointing out the US is participating in a multi-country military exercise this year and is sending a new ambassador to Bangkok next month.

The Thai Foreign Ministry issued a statement hailing a "long relationship" of more than 180 years with the United States and said it had advised Mr Russel of the government's "reform procedures."

But soon after toppling Ms Yingluck's democratically-elected government the generals turned to China after several Western nations, including the US, imposed sanctions and downgraded ties with Thailand.

The generals greeted Chinese businessmen as their first guests after the coup.

In August, the junta approved a US$23 billion plan to build two high-speed railways linking directly with China by 2021 in a move seen as a further consolidation of Chinese power in the region.

The Thai leg of the railway is a key link in China's plan to build a 3000 kilometre high-speed line from Kunming, in China's southern Yunnan province all the way to Singapore, passing through Laos, Thailand and Malaysia.

Mr Prayuth, a former army chief, cited China as a good example of economic development in a year-end speech after making a two-day visit to China.

"I spoke to Chinese president Xi Jinping and he told me that 60 years ago his country was (one of) the poorest in the world. In 30 years they were able to make their country a world economic superpower," he said.

"But we are still bickering amongst ourselves."

Some analysts saw the remarks as indicating Mr Prayuth's admiration of an authoritarian one-party rule in exchange for economic prosperity.

Thailand's economy is faltering, growing slower than some neighbours, including Malaysia, Vietnam and Indonesia, though many foreign investors have remained committed to the country following the coup.

The junta has vaguely promised elections by early 2016 but has refused to lift a 100-year-old martial law imposed before the coup that gives the military absolute power across the country, including banning political gatherings.

The generals insist they had no option but to take over to end months of sometimes violent anti-government protests.

Australia's criticism of the junta has been muted as Canberra has taken a more pragmatic view of the military's rule than the US, stressing in an Australia Day message the existence of a "broad and deep relationship."

Australia's Foreign Affairs Minister Julie Bishop is expected to visit the country soon.