Guaido says intelligence agents detained his chief of staff, Roberto Marrero, in pre-dawn raid.

Venezuelan opposition leader Juan Guaido said on Thursday intelligence agents had arrested his chief of staff following a pre-dawn raid, signaling that President Nicolas Maduro may be cracking down on the opposition’s challenge to his rule.

US Secretary of State Mike Pompeo, in a Twitter post on Thursday, called for Roberto Marrero’s immediate release and said, “We will hold accountable those involved.”

The United States has repeatedly warned Maduro not to move against Guaido.

Guaido said since Maduro’s government could not arrest the interim president, they were trying to arrest those close to him instead.

Guaido invoked the Constitution in January to assume the interim presidency after declaring Maduro’s 2018 re-election a fraud – a move Maduro has called an attempted coup.

Maduro, who has overseen a dramatic collapse of the OPEC nation’s economy, has called Guaido a puppet of the US and said he should “face justice”, but has not explicitly ordered his arrest.

“They have kidnapped @Robertomarrero, my chief of staff,” Guaido said in a post on Twitter, adding the Caracas residences of Marrero and opposition legislator Sergio Vergara had been raided before dawn.

Marrero sent a last-minute voice note as the agents were trying to enter his house, saying, “Look after the president”. The voice note was shared with reporters by Guaido’s press team.

The press team also sent a video of another opposition legislator showing damage to the door of Marrero’s home and a broken lock.

“The United States condemns raids by Maduro’s security services and detention of Roberto Marrero,” Pompeo said on Twitter.

US National Security Adviser John Bolton said the government of Nicolas Maduro had “made another big mistake” with the “illegitimate arrest” of the aide, Marrero. “He should be released immediately and his safety guaranteed,” Bolton wrote in a Twitter post.

Venezuela’s information ministry did not immediately respond to a request for comment from Reuters.

‘Economic war’

Dozens of countries including the US, major European powers and most South American nations have backed Guaido and say Maduro’s rule is illegitimate.

Maduro says his government is the victim of an “economic war” led by his political adversaries and blames US financial and oil sector sanctions for the country’s situation.

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Guaido travelled around South America in February to drum up diplomatic support for his government, defying a travel ban imposed by the Supreme Court.

He later entered the country via Venezuela’s principal airport without being arrested by immigration officials.

Since January, Venezuelan authorities have arrested more than 1,000 people in connection with anti-government demonstrations, rights groups have said.

UN human rights chief Michelle Bachelet said on Wednesday that Venezuelan security forces, backed by pro-government militias, have quashed peaceful protests with the excessive use of force, killings and torture.

Venezuela is reeling from annual inflation topping two million percent, which has fueled malnutrition and preventable disease and spurred an exodus of more than three million citizens in since 2015.