US Secretary of State John Kerry met Venezuelan President Nicolas Maduro on Monday, the first formal encounter between the two since Kerry became the top US diplomat.

The two sat down together after a ceremony in the Colombian city of Cartagena to mark the signing of a peace agreement to end that country's civil war.

"(Kerry) spoke of our concern about the economic and political challenges that have affected millions of Venezuelans, and he urged President Maduro to work constructively with opposition leaders to address these challenges," said State Department Spokesman John Kirby.

Kerry and Maduro also agreed to continue bilateral discussions that began in recent months, Kirby said.

Venezuela's Information Ministry did not immediately respond to an email seeking comment on the meeting.

Venezuela and the United States have been at loggerheaders since the government of late President Hugo Chavez, with frequent exchanges of barbs and expelling each others' diplomats. They do not have ambassadors in place following expulsions several years ago.

Maduro's government is struggling under an unraveling socialist economic system that suffers from triple-digit inflation, extensive shortages of staple products and food riots.

Maduro says the country is victim of an "economic war" backed by the United States and the Venezuelan political elite.

Venezuela's opposition called on Monday for a nationwide rally on Oct. 12 to push for a referendum to recall Maduro this year as they seek to oust his Socialist Party in an early presidential election.

The recall effort has run into opposition from the election board, which is imposing restrictions and argues it will take until 2017 to put the proper conditions in place.

"Everybody knows we're working toward this recall. It's been delayed. That is problematic," Kerry said before the meeting. "And we need to find a way forward that can provide a consensus that provides relief to a nation under siege."

Colombian President Juan Manuel Santos and FARC guerrilla leader Rodrigo Londono, better known as Timochenko, signed the peace accord on Monday at a ceremony attended by international leaders and thousands of Colombians.

Santos' predecessor, Alvaro Uribe, frequently accused the government of neighboring Venezuela of harboring and supporting the FARC.