Military rule in Thailand is likely to last longer than expected and has been more repressive than after the country's last coup in 2006, a senior U.S. official said on Tuesday.

The official told a congressional hearing Washington was still looking at whether the big regional Cobra Gold military exercise held annually in Thailand could go ahead there next year given the military takeover in May.

"Initially, we held out hope that - as happened with the 2006 coup - the military would move relatively quickly to transfer power to a civilian government and move towards free and fair elections," said Scot Marciel, the U.S. principal deputy assistant secretary of state for East Asia.

"However, recent events have shown that the current military coup is both more repressive and likely to last longer than the last one."

Marciel said in testimony to the Asia-Pacific subcommittee of the House Foreign Affairs Committee that the coup had put the United States in a difficult position, given that Thailand is a key U.S. ally in Asia.

"The challenge facing the United States is to make clear our support for a rapid return to democracy and fundamental freedoms, while also working to ensure we are able to maintain and strengthen this important friendship and our security alliance over the long term," he said.

Marciel said Washington hoped that strong international criticism of the military takeover would lead to an easing of repression and an early return to democracy. He said the United States would continue to call for martial law to be lifted and elections to be held sooner than a vague 15-month timeline laid out by the military government.

However, he added: "To be honest, it's very hard to predict how long they are going to stay in power."

Until there is a return to elected government, "we will not be able to do business as usual," Marciel said.

SHIFTING MILITARY EXERCISES

Thai Air Force Air Chief Marshall Prajin Juntong played down the significance of any possible move to shift elsewhere the U.S. and Thai-led Cobra Gold exercises, held annually in Chon Buri, a province east of Bangkok.

"The Royal Thai Air Force trains with other international friends, including Singapore, Malaysia and Indonesia. This should be no problem at all," Prajin told reporters in Bangkok.

Prajin said he was confident that relations with Washington would return to normal quickly as they did after the 2006 coup, when the United States cut aid to its military ally.

"At that time, the U.S. pressured us too, but after we created understanding the situation returned to normal and we believe it will be like that again this time."

As required by U.S. law, Washington has frozen $4.7 million of security-related assistance since the coup and canceled high-level engagements, some military exercises and training programs for the military and police.

Marciel said Washington had yet to make a decision on Cobra Gold, planned for early next year, which he called "hugely important ... not only for Thailand and the United States, but for the region."

"It's something we're looking at. We have a little bit of time to work with."

Steve Chabot, chairman of the subcommittee at which Marciel spoke, suggested that Cobra Gold could be moved to another country, such as Australia, and added: "It could clearly send the wrong message if we allowed (Thailand) to participate."

Washington has also yet to decide whether Thailand would receive a presidential waiver on sanctions - including withdrawal of U.S. support at the International Monetary Fund and the World Bank - that could be imposed what it sees as Bangkok's failure to deal with human trafficking, Marciel said.

The U.S. State Department last week downgraded Thailand to its lowest rank in a survey of countries' efforts to eliminate trafficking, placing it alongside states such as North Korea, Syria and Uzbekistan.