Bahrain has charged the country's most senior opposition leader and one of his aides with holding an illegal meeting with a US diplomat, the public prosecutor's office said on Thursday.
It said al-Wefaq party leader Sheikh Ali Salman and his political assistant, Khalil al-Marzouq, should have obtained permission before meeting Tom Malinowski, US Assistant Secretary of State for Democracy, Human Rights and Labor.
Bahrain expelled Malinowski earlier this week, saying he had "intervened flagrantly" in the country's internal affairs by holding the meeting. The United States has said it is "deeply concerned" about his treatment and is considering a response.
The incident has opened a rift between Washington and one of its main regional allies. Bahrain hosts the headquarters of the US Fifth Fleet but has bristled at American criticism over its human rights record since suppressing a popular uprising in 2011.
The public prosecutor said Salman and Marzouq were questioned and then charged with "contacting a representative of a foreign government in violation of the political associations law and related ministerial decisions".
They were freed after guaranteeing their places of residence, it added, without giving any further details.
Al-Wefaq confirmed the charges and called them unfair, saying such regulations had never been implemented before and no one had been prosecuted for them.
Malinowski attended a Ramadan evening meeting of al-Wefaq on Sunday and met Salman and an aide again at the US embassy on Monday. He said he was asked whether they had made specific requests of the Americans, and replied that they had not.
Salman and Marzouq were interrogated at the Criminal Investigations Department on Wednesday before they were summoned to appear at the public prosecutor's office on Thursday. Salman told Reuters he was questioned for about half an hour, without his lawyer, "about the content of the (embassy) meeting and what was discussed at it."
A court in Bahrain last month cleared Marzouq of terrorism charges.
The Gulf island kingdom is ruled by a Sunni Muslim royal family, but the majority of its population are Shi'ites, whose political leaders have demanded democratic reforms.
Bahraini authorities, backed by troops from Saudi Arabia, quelled demonstrations during the "Arab Spring" revolts that swept the region three years ago, but low level violence is still common.