The source said representatives of Britain, the United States, France, Russia and Germany had reached agreement with Beijing during a conference call.
"It has been agreed with China to start drawing up sanctions on Iran," the source said. "Drawing up of a Security Council resolution is to begin in the next few days."
Diplomats say China has been slowly and reluctantly falling in line with the other powers involved in the negotiations on Iran by backing the idea of new U.N. sanctions against Tehran but Beijing wants any new steps to be weak.
They say the four Western powers would like a resolution to be adopted next month, before a month-long U.N. conference on the nuclear Non-Proliferation Treaty in May, but acknowledge that negotiations will probably drag on at least until June.
World leaders accuse Iran of developing a nuclear arsenal but Tehran says its nuclear program is intended only to generate electricity.
"This is a big victory for the United States and the Europeans," said the source. "China has taken great strides."
The source gave no other details.
(Editing by Andrew Dobbie)
