The United States and more than a dozen Latin American countries on Saturday warned Venezuela its presidential election next month would be seen as illegitimate by the region unless it restored democratic standards.

The May 20 poll would be "void of legitimacy and credibility" if it went ahead under current conditions, the nations said in a joint declaration released at a Summit of the Americas in Peru.

Venezuela's opposition says President Nicolas Maduro has prepared a rigged snap election to deliver him a new mandate and tighten his hold over his economically devastated country.

US Vice President Mike Pence, representing America at the summit, said the election was a "sham."

"The United States is prepared to continue to bring all pressure to bear, working with our allies," to restore democracy in Venezuela, he told reporters.

The joint statement was signed by the US and the 16-nation Lima Group which counts Latin America's biggest economies, including Brazil, Mexico, Argentina and Chile.

Canada, whose Prime Minister Justin Trudeau expressed extreme concern over Venezuela's situation during remarks at the close of the summit, is also part of the group.

The statement demanded "necessary guarantees" for the Venezuelan poll to be recognized: a fair and transparent electoral process; the release of political prisoners; and the participation of the opposition, which has been largely excluded.

Maduro was not invited to the Peru summit. He has accused the United States of helping Venezuela's opposition to undermine his authority by waging what he calls an economic "war."

The United States has already slapped sanctions on Maduro, his officials, and Venezuela's state-owned oil company PDVSA.