Renault's stock was up 4.6 percent at 97.41 euros as of 1057 GMT, after Bloomberg said the companies were in merger discussions, citing unidentified sources. Nissan shares had earlier closed 0.5 percent lower in Tokyo.
Reuters reported earlier this month that the carmakers were discussing a deal, at Chief Executive Carlos Ghosn's initiative, that would see Nissan acquire much of the French government's 15 percent Renault holding, as a prelude to a merger.
"We do not comment on rumors and speculation," Renault-Nissan spokesman Jonathan Adashek said on Thursday when asked about the latest report. A French finance ministry spokesman also declined to comment.
The Renault-Nissan alliance, underpinned by cross-shareholdings, has wrestled intermittently since its 1999 inception with full merger proposals that have foundered on objections from France, Renault's biggest shareholder.