MOSCOW, September 26 (RIA Novosti) - President Barack Obama said in his weekly address on Saturday that the U.S. and Russia will work together on preventing the proliferation of nuclear weapons, and on reducing their own nuclear stockpiles.

The message, recorded on the president's way back from the G20 Summit in Pittsburgh, said: "The United States is meeting our responsibilities by pursuing an agreement with Russia to reduce our strategic warheads and launchers. And just as we meet our responsibilities, so must other nations, including Iran and North Korea."

Both Russia and the U.S. have voiced concerns over Iran's recent revelations on its second nuclear enrichment facility.

The president called Iran's nuclear program "a serious challenge to the global nonproliferation regime," showing a "disturbing pattern of Iranian evasion."

At the summit in Pittsburgh, "President Medvedev of Russia and I agreed that Iran must pursue a new course or face consequences. All of the permanent members of the United Nations Security Council, and Germany, have made it clear that Iran must fulfill its responsibilities," Obama said.

Iran's president has rejected Western accusations that his country's nuclear program runs counter to international agreements, and insisted on the legality of the newly revealed uranium enrichment site.

The new plant is being built near Qom, around 160 km (100 miles) southwest of Tehran.

Russia, which has traditionally supported Iran in the long-running dispute over its nuclear program, voiced concern on Friday over the second enrichment site, and urged the country to provide reassurances over its nuclear program.

President Dmitry Medvedev said in a statement that the fact that Iran has been building a uranium facility for several years without informing the UN nuclear watchdog is a "source of serious concern."

He urged the country to take steps to prove that its nuclear program is peaceful before a meeting with six world powers in Geneva on October 1.