The United States has cancelled an ongoing military exercise with Thailand and planned visits by officials after the Thai army seized power in a widely condemned coup.

Thailand's army said on Thursday that it had taken power in a coup after months of unrest and deadly political violence, provoking an international outcry and heightening fears for the future of the Asian country and its fragile democracy.

"While we have enjoyed a long and productive military-to-military relationship with Thailand, our own democratic principles and US law require us to reconsider US military assistance and engagements," Pentagon spokesman Rear Admiral John Kirby said in a statement.

The Cooperation of Afloat Readiness and Training exercise (CARAT), which involves several hundred US Marines and sailors, began Monday and was supposed to run for a week.

The scrapped visits had been planned for June - one to Thailand by US Pacific Fleet Commander Admiral Harry Harris and another to the US Pacific Command in Hawaii by a high-ranking Thai military official.

"It is important that the Royal Thai Armed Forces end this coup and restore to the people of Thailand both the principles and the process of democratic rule, including a clear path forward to elections," Kirby said.

"We urge the Royal Thai Armed Forces to act in the best interests of their fellow citizens by ending this coup and restoring the rule of law and the freedoms assured those citizens through democratic principles."

The US military has long-standing ties to Thailand's armed forces, dating back to the Korean and Vietnam wars.

Still, the Pentagon also warned of potentially more cuts.

"We will continue to review additional engagements as necessary until such time that events in Thailand no longer demand it," Mr Kirby said.

Word of the cancellations comes on the heels of an announcement by Washington on Friday that it had suspended $3.5 million in military assistance to its oldest treaty-bound ally in Asia.

The US State Department meanwhile said that the United States was reviewing the rest of its aid to Thailand -- which totalled $10.5 million in 2013.

In a statement, State Department spokeswoman Marie Harf said Washington had now also called off a US government-sponsored firearms training program for the Royal Thai Police that had been scheduled to start Monday.

"We are increasingly concerned about actions the military has taken, just a few days after it staged a coup," Ms Harf said, pointing to the dissolution of the Senate, arrests and media restrictions.

"We again call on the military to release those detained for political reasons, end restrictions on the media and move to restore civilian rule and democracy through elections."

Yingluck in 'safe place' - Thai military

Former Thai Prime Minister Yingluck Shinawatra was in a "safe place", an aide said, after being held by the army following the coup.

The military detained Ms Yingluck yesterday when she and scores of other people, most of them political associates, were summoned to an army facility in Bangkok.

More people have been summoned this weekend, including some academics.

A senior officer said Ms Yingluck could be held for up to a week and media reported she had been taken to an army base in Saraburi province north of Bangkok, but an aide denied that.

"Now she's in a safe place ... She has not been detained in any military camp. That's all I can say at this moment," the aide said, declining to be identified.

A source from her Puea Thai Party added: "We can't say she is absolutely free because there are soldiers in the area, monitoring her."

The source said several former ministers from her cabinet were being held in army facilities in Saraburi.

Army deputy spokesman Winthai Suvaree told a news conference that anyone being held would not be detained for more than seven days. He did not mention Ms Yingluck.

Thailand's political woes are the latest chapter in a nearly decade-long clash between the Bangkok-based establishment and Thaksin Shinawatra, a former telecommunications tycoon whose populist policies won him huge support and repeated electoral victories.

Mr Thaksin was ousted in a 2006 coup and left the country after a 2008 graft conviction, but he was the guiding hand behind the government of his sister Ms Yingluck.

Despite international calls for the restoration of democratic government, army chief General Prayuth Chan-ocha has not promised a swift return to civilian rule, insisting there must be broad reforms and stability first.

Human Rights Watch said rights in Thailand were in "freefall".