"We will continue to increase pressure on the corrupt regime, support the democratic National Assembly, and call for democracy and freedom in Venezuela," he wrote.
Maduro is being sworn in for a second six-year term under a cloud of skyrocketing inflation, shortages of basic food and medicine and an exodus of Venezuelans to neighboring countries, following an election that was boycotted by the opposition.
The Lima Group -- a bloc of 14 Latin American powers and Canada -- urged Maduro to renounce his term and hand over power to parliament, although Mexico's new leftist government stayed neutral.
The European Union has also said that last year's election was marred by fraud and has extended sanctions. The United States has for years put pressure on Maduro and his predecessor Hugo Chavez, with President Donald Trump publicly musing about military intervention.
On Tuesday, the United States imposed its latest sanctions as it targeted seven Venezuelans allegedly involved in black-market currency exchanges that generated billions of dollars in illicit profits.
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