US Secretary of State Mike Pompeo has demanded Venezuela reopen a bridge on the Colombian border for an aid shipment organised by the opposition.
Venezuelan soldiers have blocked the crossing ahead of a delivery arranged by opposition leader Juan Guaidó, who has declared himself interim president.
President Nicolás Maduro, who has the support of the army, has rejected letting it into the country.
In a tweet, Mr Pompeo demanded Mr Maduro let the aid through.
“The Maduro regime must LET THE AID REACH THE STARVING PEOPLE,” the post reads.
Venezuela is in the grip of a worsening humanitarian crisis.
Mr Guaidó is head of Venezuela’s National Assembly and says the constitution allows him to assume power temporarily when the president is deemed illegitimate.
He has secured the backing of over 40 countries, including the US and most Latin American and European nations. Mr Maduro still has the support of China and Russia.
What is happening at the border?
The Venezuelan military had used a tanker truck and a cargo container to block access to the Tienditas bridge which links Cucuta, Colombia to Urena, Venezuela, opposition politician Franklyn Duarte told AFP news agency.
“Troops from the armed forces are blocking the pass”, he said.
Other opposition leaders appealed to the military to allow aid trucks to cross the border.
“You know there’s a red line, you know well there’s a limit, you know that medicines, food and medical supplies are that limit,” said lawmaker Miguel Pizarro.
Pictures shared on social media appeared to show residents confronting soldiers in Urena.
Mr Maduro said humanitarian aid would be the start of a US-led invasion, insisting that “no-one will enter, not one invading soldier”.
How will the aid scheme work?
Mr Guaidó does not control any territory in Venezuela so, instead, he is planning to set up collection centres in neighbouring countries where Venezuelans have fled to.
He said he wanted to set up an international coalition to gather aid at three points, and press Venezuela’s army to let it into the country.
In a tweet on Sunday, US National Security Adviser John Bolton said plans were being advanced.
In his State of the Union speech, he reiterated his support for Mr Guaidó, saying: “We stand with the Venezuelan people in their noble quest for freedom.”