The four Western powers trying to rein in Iran’s nuclear program have accused Tehran of shipping arms to Syria in violation of UN sanctions and ignoring demands to open key nuclear facilities to UN inspectors.

The United States, Britain, France and Germany on Thursday expressed growing concern that Iran’s goal is building a nuclear arsenal, not nuclear power plants for peaceful civilian use, as Tehran insists.

US Ambassador Susan Rice told a security council meeting on the implementation of UN sanctions that members can’t be complacent about Iran’s “latest leaps forward in its prohibited nuclear activities”.

“We must recognize that we are facing a situation that continues to worsen,” she said.

A spokesman for Iran’s mission to the UN did not immediately respond to an email seeking comment.

Rice and ambassadors from the other Western powers expressed serious concern at Iran’s arms exports to President Bashar Assad’s regime in violation of a UN ban against all weapons exports.

She reiterated the conclusion of the panel of experts monitoring implementation of sanctions that Syria is now the “central party to illicit Iranian arms transfers”.

The 18-month Syrian conflict has escalated into a civil war which has killed more than 23,000 people, according to activists.

Russia and China, who have vetoed three Western-backed resolutions aimed at pressuring the Assad regime to end the violence and start talks with the opposition, made no mention of Iranian arms shipments to Syria in their speeches to the council but did address the nuclear issue.

On Iran’s nuclear program, the Western powers expressed alarm at the latest report from the International Atomic Energy Agency.

It said Iran was effectively shutting down a probe of a site suspected of being used for work on nuclear weapons development while doubling the number of machines it could use to make the core of nuclear warheads.

Nuclear concerns will be at the top of the agenda at a ministerial meeting of the four Western powers, Russia and China on September 27, on the sidelines of the annual General Assembly that begins on Tuesday.