US Secretary of State Mike Pompeo has demanded Venezuela open 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.
The Venezuelan people desperately need humanitarian aid. The U.S. & other countries are trying to help, but #Venezuela’s military under Maduro's orders is blocking aid with trucks and shipping tankers. The Maduro regime must LET THE AID REACH THE STARVING PEOPLE. #EstamosUnidosVE pic.twitter.com/L4ysYJaM6H— Secretary Pompeo (@SecPompeo) February 6, 2019
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 cargo containers to block access to the Tienditas bridge which links Cúcuta, Colombia to Ureña, Venezuela, opposition politician Franklyn Duarte told AFP news agency.
"Troops from the armed forces are blocking the pass", he said. The bridge, a joint project between Colombia and Venezuela, was finished in 2016 but never opened as relations between the two neighbours had soured.
Venezuelan 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 Ureña.
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."
Correction 25 February 2019: This article has been amended to make clear that while construction of Tienditas Bridge was completed in 2016, it was never opened due to worsening relations between Venezuela and Colombia.