A top U.S. diplomat on Wednesday pressed Europe to slap new sanctions on Iran over its nuclear program, stepping up a push by Washington to hold Tehran accountable for defying U.N. demands.

A week after the United States announced new sanctions against Iran, Undersecretary of State Nicholas Burns met with French officials to press for other Western allies to follow suit, or at least loosen their economic ties with the Islamic republic.

Burns also faulted China and Russia, which have strong economic interests in Iran, for stalling progress toward a third set of Security Council sanctions. He said China was increasing its trade with Iran, "which of course is a very negative development."

Burns said military action against Iran was "not inevitable, and not desirable," and suggested that tougher penalties would be a way to avoid it.

"If we want diplomacy to succeed, we're going to have to see more tough-minded diplomacy," he told reporters. "We are going to have to see additional efforts by Europe to support diplomacy through sanctions."

European concerns have been mounting that the United States might attack to prevent Iran from developing nuclear weapons. France, which has sounded increasingly stern on Iran in recent months, and Britain have called for stronger European sanctions — but the rest of the EU remains divided on how to defuse the standoff and avoid war.

Burns said he was heading to Vienna on Thursday to meet with Mohamed ElBaradei, the head of the U.N.'s nuclear watchdog, before traveling to London for a meeting a day later of diplomats from the five permanent U.N. Security Council member countries — Britain, China, France, Russia and the United States — plus Germany.

Beyond new sanctions from Europe, Burns also called on two major U.S. allies in Asia — Japan and South Korea — to reduce their economic ties with Tehran.

The International Atomic Energy Agency began a third, final round of talks in Tehran on Monday aimed at resolving remaining questions on centrifuges that are used to enrich uranium. Those talks will provide a basis for an important progress report on Iran by ElBaradei planned for mid-November.

"We appreciate the work that he is doing, but we want him to be supportive," Burns said of ElBaradei, adding that "the Security Council has spoken" when it comes to Iran.

The council first imposed sanctions on Dec. 23, ordering all countries to stop supplying Iran with materials and technology that could contribute to its nuclear and missile programs, and freeze assets of 10 key Iranian companies and 12 individuals related to the programs.

In March, the council imposed moderately tougher sanctions including banning Iranian arms exports and freezing the assets of 28 people and groups involved in Iran's nuclear and missile programs.

Both times, Iran responded by expanding enrichment. Tehran insists its nuclear program is peaceful and designed to generate electricity for civilian use. Many in the West fear it masks a plan to develop weapons.

In the March resolution, the Security Council agreed to revisit sanctions if Tehran did not comply within 60 days — "and that was six months ago," Burns said. "It's time to move forward now."

Britain and France have been leading the drive in the 27-nation European Union to reach new sanctions on Iran, but EU foreign ministers in mid-October failed to agree on new penalties. Italy, which is Iran's biggest EU trading partner, and Germany are more reticent, calling for continued work through the United Nations.