President Nicolas Maduro of Venezuela is backing Bernie Sanders in the US election, but says America's "archaic" electoral system may prevent his "revolutionary friend" from becoming president.

Supporters of Bernie Sanders have shouted for months that their candidate is being robbed of the Democratic presidential nomination because of an unfair election process.

Those claims are coming not only from California, which will hold a crucial primary election on Tuesday, as it turns out, but also from Caracas.