SEOUL, South Korea — A senior American diplomat urged South Korea on Monday to suspend imports of Iranian petrochemicals as Washington tried to rally international pressure on Iran to stop its nuclear program.

“The situation in Iran has become more and more worrisome over recent months,” the diplomat, Robert J. Einhorn, said at a news conference in Seoul. “The timeline for Iran’s nuclear program is beginning to get shorter.”

Mr. Einhorn, the State Department’s special adviser for nonproliferation and arms control, said that if a diplomatic solution was not found, “pressures will grow for a much stronger kind of action.”

“That’s why we need to act soon,” he said. “That’s why we want to work with all of our partners around the world, including the Republic of Korea, to send a right signal to Iran.”