The United States slammed Venezuelan regional elections on Monday, after President Nicolas Maduro's socialist party claimed a resounding victory in a poll marred by a lack of outside observers and alleged manipulation.

"We condemn the lack of free and fair elections yesterday in Venezuela. The voice of the Venezuelan people was not heard," State Department spokeswoman Heather Nauert said.

"Our previously stated concerns were unfortunately realized: lack of independent, credible international observers; lack of technical audit for the National Electoral Council's (CNE) tabulation; last minute changes to polling station locations without public notice; manipulation of ballot layouts; and limited availability of voting machines in opposition neighborhoods.