Russian President Dmitry Medvedev on Friday urged Iran to prove it is not seeking to develop atomic weapons, stepping up pressure on Teheran and signaling increased solidarity with Western nations determined to rein in its nuclear program. Medvedev said revelations that Iran has secretly been building a uranium-enrichment plant raised "serious concerns," and demanded Teheran come to the table at an international meeting next week with a cooperative attitude and evidence of peaceful intentions. "We are counting on Iran - particularly in light of the newly revealed information about the construction of a new enrichment plant - to provide convincing evidence of it intention to seek to develop nuclear energy with purely peaceful aims," Medvedev said in the statement. His remarks, in a statement released during a G-20 summit overshadowed by worries over Iran, added to indications that Russia could support tougher sanctions if Teheran fails to comply with demands it come clean and halt uranium enrichment. But Medvedev did not mention sanctions or any other consequences Iran could face if it refuses to fall in line. Medvedev said the undeclared construction of an enrichment facility flies in the face of UN Security Council demands for Iran to stop uranium enrichment at its only declared enrichment facility. He suggested the UN nuclear agency should take steps immediately to investigate the second site and called for Iran's "full cooperation" with the probe. Iran is under three sets of Security Council sanctions for refusing to freeze enrichment at what had been its single publicly known enrichment plant, which is being monitored by the International Atomic Energy Agency. Russia and China, both permanent Security Council members with veto power, approved those sanctions but have stood in the way of stronger measures. Russia, which has close ties with Iran and is building the nation's first nuclear power plant, has repeatedly said it has seen no evidence to support US assertions that Iran is seeking nuclear weapons, and has warned that too much punishment would be counterproductive. But Medvedev has displayed more willingness to cooperate since US President Barack Obama announced this month that he was shelving plans for an Eastern European missile shield that deeply angered the Kremlin. After a meeting with Obama on Wednesday at the UN, Medvedev suggested Russia could sign on to stronger sanctions if Iran remains recalcitrant. The revelation about the construction of a second facility "only strengthens our determination to achieve concrete and verifiable results" in the years-long international efforts for clarity on Iran's nuclear program and its goals, Medvedev said. Russia has been among the nations leading those efforts - along with the US, France, Britain, Germany and China - but has been careful not to ruin ties with Teheran by lining up more solidly with the West. Medvedev's statement came after Obama and the leaders of France and Britain said Teheran must quickly disclose all its nuclear efforts - including any moves toward weapons development - "or be held accountable." French President Nicolas Sarkozy said Iran has until December to comply or face new sanctions. Representatives of the six nations are to meet with Iranian officials in Geneva on October 1. Medvedev stressed that Russia is "committed to a serious dialogue with Iran" and the meeting is a chance for Teheran to show it is determined to seek a negotiated solution to the dispute over its nuclear program - another hint that Moscow could support new sanctions if that doesn't happen.