Serkan Golge, a naturalized US citizen working for the US space agency in Houston, was arrested in July 2016 on a visit back to Turkey in the aftermath of a failed coup against President Recep Tayyip Erdogan.

Turkish authorities charged Golge with ties to self-exiled cleric Fethullah Gulen, whom Erdogan accused of orchestrating the mutiny. Golge was sentenced in 2018 to seven and a half years in prison despite US State Department protests that there was no credible evidence.

His wife, Kubra Golge, expressed joy at his release but said that he remained banned from traveling outside Turkey.

"We are happy but he still rejects the charges against him," she told AFP by email. "Hope we can come back soon to the US."

State Department spokeswoman Morgan Ortagus said that the United States would press for Golge to be able to return to the United States "as soon as possible."

"We want to commend them for doing the right thing today by releasing him," Ortagus told reporters. "We think it's welcome news."

Ortagus said that the United States was still seeking the release of detained local employees of US diplomatic missions in Turkey.

Golge was freed shortly after Erdogan spoke by telephone with US President Donald Trump, although an official summary by Turkey did not mention discussion of the Golge case.

Turkey in October also released an American pastor caught up in the crackdown, Andrew Brunson. His case had become a cause celebre among the conservative Christian base of Trump, who pressed Turkey through tariffs that sent the lira currency into a tailspin.

- Rift over missile deal -

Golge's case had triggered growing anger in the United States. A bipartisan group of senators recently introduced a bill seeking sanctions on Turkish officials involved in the detention of US citizens, saying that Ankara's actions did not befit a NATO ally.

But Turkey still is at risk of US sanctions over its purchase of Russia's S-400 missile defense system. Washington is pressing Ankara to instead buy the US Patriot equivalent.

Erdogan has said the S-400 purchase was a "done deal" but in the phone call with Trump reiterated an offer to form a joint working group on the decision, according to the Turkish president's office.

The State Department voiced appreciation for Turkey as an ally but reiterated concerns about the deal, which US officials say could help Russia hone its system to target US hardware used by NATO.

"We're willing to engage with the Turkish government but our position remains the same that Turkey will face very real and very negative consequences if it completes the delivery of the S-400," Ortagus said.

The United States has already suspended Turkey's participation in the F-35 fighter jet program, in which Turkey had invested $1 billion.

Washington and Ankara have also clashed over Syria, with Trump promising to pull out all 2,000 US troops from the war-battered nation following a December phone conversation with Erdogan.

Trump has since slowed down the withdrawal partly because his aides fear that Turkey will use the absence of US troops to strike Syrian Kurdish fighters. The forces helped defeat extremists from the Islamic State group, but Ankara associates them with separatists at home.