Venezuela's president is ordering the expulsion of three US consular staff, accusing them of infiltrating the country's universities.
Nicolas Maduro made the announcement during a televised speech amid rising tension in Venezuela over anti-government protests.
Mr Maduro did not identify the officials but accused them of infiltrating universities under the cover of consular work involving student visas.
The president has accused the US of working with the opposition in trying to topple his socialist government, a claim Washington denies.
Mr Maduro also said Venezuela's ambassador to the Organisation of American States in Washington received a phone call from the US State Department warning that the arrest of opposition leader Leopoldo Lopez would have negative international consequences for his government.
The president said he would not tolerate "threats" to Venezuela's sovereignty.
Mr Lopez, the target of a police manhunt for allegedly inciting violence at anti-government protests that ended with three deaths, has said he will surrender after staging one more demonstration.
In a video shot in an undisclosed location, he said he did not fear arrest but accused authorities of trying to violate his constitutional right to protest against Mr Maduro's government.
He urged supporters to gather in white shirts and march peacefully with him to the Interior Ministry in Caracas tomorrow , where he said he would deliver a petition demanding a full investigation of the government's role in the deaths and turn himself in to authorities.
"I haven't committed any crime," said Mr Lopez, who has not been seen since a news conference on Wednesday night after the bloodshed. "If there is a decision to legally throw me in jail I'll submit myself to this persecution."
His comments came after security forces raided his home and that of his parents on Saturday night, seeking to serve an arrest order on charges ranging from vandalism of public property to terrorism and murder.
Mr Lopez was not at either home in Caracas' leafy eastern district when national guardsmen and military intelligence officials arrived. Aides said neighbours banged on pots and pans to protest at what they considered an arbitrary detention order.
The raids capped another night of protests during which security forces fired tear gas and rubber bullets to break up a group of about 500 students who vowed to remain on the streets until all arrested anti-government demonstrators are released. Authorities said 23 people were being treated for injuries, none of them life-threatening.
More protests were held yesterday.
Mr Lopez, 42, a Harvard-educated former mayor, is the most prominent of a group of opposition hardliners challenging two-time presidential candidate Henrique Capriles for leadership of the anti-Maduro movement.
Mr Maduro accuses Mr Lopez of leading a US-backed "fascist" plot to oust him from power just two months after the ruling party's candidates won mayoral elections by a landslide. He "ordered all these violent kids, who he trained, to destroy the prosecutor's office and half of Caracas and then goes into hiding", Mr Maduro told thousands of supporters at a pro-government rally on Saturday.
"Turn yourself in, coward," he said.
Mr Lopez has vowed to press ahead with demonstrations calling for Mr Maduro to give up power. The opposition blames the socialist president for Venezuela's rampant crime, 50% inflation and worsening shortages of basic goods.
He has called on Venezuelans to avoid violence and says he had nothing to do with Wednesday's clashes between activists and police and pro-government militias after the peaceful conclusion of a rally he helped organised against the 10-month-old government.
The US has denied any plotting to oust Mr Maduro. Secretary of state John Kerry expressed concern on Saturday over the rising tensions and violence surrounding the protests.
"We are particularly alarmed by reports that the Venezuelan government has arrested or detained scores of anti-government protesters and issued an arrest warrant for opposition leader Leopoldo Lopez," he said.
"These actions have a chilling effect on citizens' rights to express their grievances peacefully."
In an apparent bid to dampen anti-government demonstrations, which have been held off and on since Wednesday, Mr Maduro said he had ordered the suspension of tube and bus services in the Chacao area of the capital where the protests are centred.
"We can't have a moment of weakness because we are trying to defeat a fascist movement that wants to end the country we have," said Maduro, the hand-picked successor to the late Hugo Chavez.
Since Friday, student protesters have disrupted traffic on the main road through Caracas for several hours each day.
"We are not going to give in or kneel. We are going to continue in the streets, fighting for Venezuelans and the youths who want a democratic country, with free media that aren't censored or self-censored, with justice and equity," said Juan Requesen, a student leader at the Universidad Central de Venezuela.