"All we have done from the beginning was help the process", he said, referring to Venezuela's internal political dialogue between the Maduro administration and opposition.
Although in cases such as this the person declared to be persona non grata must leave the country, the statement issued by Caracas does not expressly order Silva's expulsion or set any deadlines for that to occur.
Socialist party chief Diosdado Cabello has said that elections are the best way to counter criticism by the US and others that Venezuela is descending into dictatorship.
Foreign governments were quick to condemn the move, saying it undercuts the negotiations that have been taking place on and off since December.
The coalition had planned to hold primaries to settle on a joint candidate.
Although polls say Venezuelans overwhelmingly blame Maduro for widespread food shortages and quadruple-digit inflation, Maduro heads into the race the clear favorite against an opposition divided and rudderless following the collapse of a deadly protest movement previous year seeking the president's removal.
Critics of the 55-year-old Maduro, who succeeded Hugo Chavez in 2013, say he has ruined a once prosperous oil economy, turned Venezuela into a dictatorship and skewed the election system to perpetuate power for his Socialist Party. Others have fled into exile. Venezuela said on Thursday it was expelling the Spanish ambassador from Caracas in response to Spanish "aggressions", Reuters said.
European Commission spokeswoman Catherine Ray said the decision to impose travel bans and asset freezes on the seven Venezuelan officials accused of human rights abuses or breaching the rule of law was made unanimously the EU's 28 member countries. Santos had previously told Caracol Radio that he doesn't think anyone will recognize the results until there are sufficient guarantees for a transparent election. Government supporters say the opposition doesn't have enough popular support to defeat Maduro at the ballot box, leaving its only option to be rallying global allies to force him from office.