The United Nations, under intense pressure from the U.S. and other countries, withdrew an invitation to Iran to participate in a Syria peace conference this week, a diplomatic bungle that muddied international efforts to end the civil war.

The bruising international face-off over Iran's participation came just two days before world powers gather in Switzerland for a long-awaited conference aimed at finding a way out of the nearly three-year conflict that has claimed more than 100,000 lives.

Beside calling attention to international friction over Syria, the political discord exposed challenges the U.S. is facing as its pursues a rapprochement with Tehran's hard-line Islamic leadership.

Iran is the main military and financial supporter of Syrian President Bashar al-Assad's regime. Even as the U.S. pushed back on Iran's participation in the Syria talks, Iran took concrete steps Monday, verified by U.N. nuclear inspectors, to rein in its nuclear program in line with an interim agreement with the U.S. and other global powers reached in November.

Senior U.S. officials on Monday said they were committed to keeping negotiations over Iran's nuclear program apart from efforts to end Tehran's support for Mr. Assad's government.