By Caren Bohan

WASHINGTON (Reuters) - President Barack Obama said on Saturday the discovery of a secret nuclear plant in Iran showed a "disturbing pattern" of evasion by Tehran which added urgency to its October 1 talks with world powers.

Iran acknowledged the existence of the uranium enrichment facility near Qom for the first time on Monday in a letter to the International Atomic Energy Agency. U.S. officials said the disclosure was aimed at pre-empting an announcement by Western governments, which were aware of the site.

"This is a serious challenge to the global nonproliferation regime and continues a disturbing pattern of Iranian evasion," Obama said in his weekly radio and Internet address.

"That is why international negotiations with Iran scheduled for October 1 now take on added urgency," Obama said of the talks between Iran, the United States and five other powers due to occur next week in Geneva.

In meetings this week on the sidelines of the United Nations and the Group of 20 summit in Pittsburgh, Britain, France and Germany have joined the United States in raising the specter of new sanctions against Iran if it does not take steps to address concerns about its nuclear program. Russia also signaled a greater willingness to go along with sanctions.

The West accuses Iran of seeking to acquire a nuclear weapon, but Tehran insists its nuclear program is aimed at generating electricity.

Obama, who sought to engage Iran after taking office in January, said he remains committed to dialogue but has said if Iran does not respond, sanctions must be considered.

"My offer of a serious, meaningful dialogue to resolve this issue remains open," Obama said. "But Iran must now cooperate fully with the International Atomic Energy Agency and take action to demonstrate its peaceful intentions."

(Editing by Paul Simao)