President Donald Trump sternly warned Maduro's "dictatorship" that he holds him personally responsible for the health and safety of the two men.
A more severe measure would be to sanction the oil industry that brings in most of Venezuela's foreign revenue.
Maduro said Monday evening he had no intention of deviating from plans to rewrite the constitution and go after a string of enemies, from independent Venezuelan news channels to gunmen he claimed were sent by neighboring Colombia to disrupt the vote as part of an worldwide conspiracy led by the man he calls "Emperor Donald Trump". "I don't listen to orders from the empire, not now or ever".
With so much at stake, a full embargo is probably near the bottom of the Trump administration's battle plan, said Clearview's Book.
Over 8 million people voted in Venezuela's Constituent Assembly election Sunday - a turnout of over 41 percent of the electorate, according to electoral authorities - to choose the 545 members of the legislative superbody from among 6,120 candidates, in a mass display of support for a peaceful resolution to the crisis.
'The result (of the election) is unappealable.
The assembly will have powers to rewrite the constitution, and is viewed by opponents as a power-grab by President Maduro to seize control of the state institutions that oppose him.
Antonio Mugica told reporters in London on Wednesday that there was a discrepancy of 1 million votes between the turnout figures announced by the government and those recorded by his systems. Most estimates have put turnout between 2 and 4 million people.More news: Jaguar E-Pace Performs World's Longest Barrel Roll
"The European Union and its member states, therefore, can not recognize the Constituent Assembly as they have concerns over its effective representativeness and legitimacy and call on the government of President Nicolas Maduro to take urgent measures to rectify the course of events".
Maduro disputed the assertion Wednesday.
The opposition, which boycotted the vote, has dismissed the official tally of 8.1 million participants as fraudulent.
Latin American nations including Chile, Colombia, Mexico and Peru joined the U.S. in saying they did not recognise the results of Sunday's election, while Brazil, Costa Rica and Panama joined the condemnation of Lopez and Ledezma's arrest.
She said she has been consistent in her political views, while others in government have changed theirs.
Maduro has dismissed the U.S. sanctions and criticism, retorting that he will not heed "imperial orders".
In July 2016, a series of leaked emails showed that during her time as Secretary of State, Hillary Clinton promoted subversion against the Latin American country, despite public appeals for friendship - she asked then-assistant Secretary of State for Western Hemisphere Affairs, Arturo Valenzuela, how best to "rein in" Chavez, who suggested reaching out to other regional partners to help undermine his leadership. Maduro was now the fourth member of an exclusive club of sanctioned dictators, joining Syria's Bashar al-Assad, North Korea's Kim Jong Un and Zimbabwe's Robert Mugabe. Other heads of state now subject to USA sanctions include Zimbabwe's Robert Mugabe and North Korea's Kim Jong Un.
Late on Wednesday, Maduro said he would postpone the first meeting of the newly formed constitutional assembly from Thursday to Friday.
The drop in global energy prices and political corruption have destroyed Venezuela's oil-rich economy.
Venezuela's opposition is readying a new showdown with President Nicolas Maduro, rescheduling a massive protest to coincide with his inauguration of a powerful new assembly set to replace congress.