Venezuelan opposition leader Henrique Capriles says he has been stopped from leaving the country to meet UN officials in New York.
As protests against the leftist government continued in the capital Caracas, he said his passport had been seized at the airport and would not be returned until 2020.
US President Donald Trump has described Venezuela’s crisis as a “disgrace”.
The US treasury has put eight supreme court members on a financial blacklist.
It called this punishment for undermining the country’s democratically elected congress by assuming its powers in late March. A senior official in Washington said further action would be taken if there was no improvement in Venezuela following weeks of worsening instability.
At least 43 people have died in the last seven weeks in violence related to the anti-government protests.
The opposition and the government accuse each other of trying to stage a coup.