Venezuela's opposition leader Guaido attends a protest of the public transport sector against the government of Venezuela's President Nicolas Maduro in Caracas. Photo: Manaure Quintero/Reuters.

Mexico City - The Venezuelan opposition is trying to freeze Swiss bank accounts held by Venezuelan officials after "irregular movements" were discovered, self-declared interim president Juan Guaido said Wednesday.

Guaido told Mexican broadcaster Televisa he had discussed the matter with Swiss President Ueli Maurer on the phone.

He said there had been attempts to "irregularly" move funds to private bank accounts, adding: "[We] are trying to freeze all those assets, which belong to the republic."

Guaido, who is demanding that President Nicolas Maduro step down, has been trying to get control over Venezuelan assets abroad.

Maduro, however, is holding on to power and rejects plans by the US-backed opposition to bring humanitarian aid into the country, which is suffering from widespread food and medicine shortages.

Maduro says Venezuela needs no aid and that such plans are a plot to stage a US military intervention and to oust him from power.

Venezuela on Tuesday closed air and sea routes to aid transport hub Curacao and to fellow Dutch islands Aruba and Bonaire.

The outcome of the stand-off is expected to depend on the army, which has so far sided with Maduro, but which the opposition is trying to persuade to let the aid through.

The commander of the US Southern Command warned the Venezuelan army that it will be "held accountable" for its actions.

"Do the right thing," Craig Faller said on a video posted on Twitter by the television channel TV Venezuela Noticias on Wednesday. "Save your people and your country."