CARACAS, Venezuela -- Venezuelan leader Juan Guaido said Friday that the opposition's demand for presidential elections is not negotiable, slowing mediation efforts by Norway aimed at resolving Venezuela's political crisis.

"A new meeting isn't planned at the moment, until we can get what we've proposed on the agenda," said Guaido at a rally in the central city of Valencia.

Guaido's comment shows how hard it will be to negotiate a peaceful solution to the crisis in Venezuela, which has endured economic and political turmoil for years. Past talks between the government of President Nicolas Maduro and the opposition have collapsed, deepening the acrimony between the two sides.

The government of Maduro did not immediately reply but has previously suggested that elections be held for the opposition-controlled congress, rather than for the presidency.

Russia's state news agency Tass quoted Alexander Shchetinin, the Russian foreign ministry's director for Latin America, as saying that he understood another round of Norway-mediated talks would happen next week.

"But this is not a question for us, but for the Norwegian intermediaries," Shchetinin said on the sidelines of an economic forum in St. Petersburg.

Norway has hosted two rounds of exploratory talks in Oslo between the Venezuelan government and opposition in the past month. While little about the talks has been announced, the opposition has insisted the starting point for negotiations be a willingness by Maduro to hold elections within a reasonable time frame.

"As long as both sides are hurting and don't see a way out, there's a possibility negotiations can succeed," said James Dobbins, a senior fellow at the Rand Corporation who served as special U.S. envoy to several crisis hotspots including Haiti and Afghanistan. "It's really the only hope left."

The setback to what appeared to be the best chance for peacefully resolving Venezuela's power battle comes amid broad shuttle diplomacy efforts in the region.

Cuban Foreign Minister Bruno Rodriguez was travelling Friday to Toronto for talks on Venezuela with Canadian Foreign Minister Chrystia Freeland, hours after meeting Venezuelan socialist party boss Diosdado Cabello, who travelled to the island.

Freeland, whose country has joined the Trump administration in pressuring Maduro to resign, said they will "discuss the role that Cuba can play in a peaceful resolution to the crisis in Venezuela."

Cabello arrived in Cuba on Thursday for what local media called discussions about a forum in Caracas next month. One of Cabello's first meetings was with Rodriguez, who said on Twitter they "discussed themes of international interest."

Guaido, who heads the opposition-controlled congress, revived a flagging opposition movement in January by declaring himself Venezuela's rightful leader, quickly drawing recognition from the United States and more than 50 nations that say Maduro's re-election last year was illegitimate.

But Maduro, with the support of the military and allies Cuba and Russia, has held on to power in the face of U.S. oil sanctions that are adding to misery in a nation hit hard by hyperinflation and widespread fuel, food and power shortages.