US Secretary of State Mike Pompeo on Wednesday welcomed Egypt's release of a US citizen but urged the ally to stop harassment of others.

Mohamed Amashah, 24, was freed Monday, nearly 16 months after he was arrested in Cairo's Tahrir Square for holding up a sign seeking the release of prisoners, according to human rights campaigners.

A dual US-Egyptian citizen who lives in New Jersey, he had gone on a hunger strike this year to protest his conditions.

"We thank Egypt for securing his release and his repatriation," Pompeo told a news conference.

"But at the same time, we urge Egyptian officials to stop unwarranted harassment of US citizens and their families who remain there," he said.

Another US citizen, Mustafa Kassem, died in Egyptian custody in January from a hunger strike. He was rounded up on a visit to Cairo in 2013 as part of a sweeping crackdown.

President Donald Trump's administration has formed close ties with Egypt's general turned president, Abdel Fattah al-Sisi, praising his fight against Islamic militants and his support for a peace treaty with Israel.

In an open letter last week, a group of foreign policy experts urged Pompeo to tell Egypt that cooperation including military aid will be at stake unless the government ends harassment of US citizens along with Egyptian activists and journalists.

These experts pointed to the continued detention of Americans including Khaled Hassan, who was allegedly tortured, and reprisals against the family of a man who brought a suit in a US court.

"As the Trump administration tries to hold Iran and China to account for their reckless and dangerous abuses of human rights, we hope that you will hold Egypt to the same standards," said the working group on Egypt organized under the Carnegie Endowment for International Peace.

"Continued mass violations of human rights increase the prospect of instability in Egypt and threaten US national security interests," they wrote.