US President Barack Obama on Wednesday issued a stern warning to Venezuela's government to not block "legitimate" efforts by the opposition to hold a referendum to recall the country's embattled president Nicolas Maduro.

"The democratic process should be respected," Obama said at a North American summit as he stood alongside Canadian Prime Minister Justin Trudeau and Mexican President Enrique Pena Nieto.

"And that includes legitimate efforts to pursue a recall referendum consistent with Venezuelan law," Obama added.