PITTSBURGH/VIENNA: US President Barack Obama and other Western leaders accused Iran on Friday of building a secret nuclear fuel plant and demanded Teheran immediately halt what he called a "direct challenge" to the international community.

Obama went public with the charge in an appearance with British Prime Minister Gordon Brown and French President Nicolas Sarkozy at a Group of 20 summit in Pittsburgh, sharpening a standoff with Iran over its disputed nuclear program. Russia later also expressed alarm.

"It is time for Iran to act immediately to restore the confidence of the international community by fulfilling its international obligations," Obama said, adding that Teheran had been building the plant in secret for years.