TEHRAN, Iran – Iran’s economic ally China urged all sides Tuesday in upcoming nuclear talks to show “flexibility and sincerity” even as Tehran sent mixed messages with hints of compromise and blasts of defiance.

The appeals by China carry significant weight in Tehran and the West, and appeared aimed at nurturing the cautious hope that this dialogue won’t quickly collapse like the last attempt more than a year ago.

Senior envoys from Iran and six world powers – the five permanent U.N. Security Council members plus Germany – are expected to hold preliminary strategy sessions Friday in Istanbul and move into direct talks the next day.

China provides an economic lifeline to Iran as one of its major oil customers and investors in Iranian projects. China and Russia, also part of the nuclear talks, have opposed the tough sanctions implemented by the U.S. and Europe.

The U.S. and allies are rallying around efforts to quickly curb Iran’s uranium enrichment, a process which produces reactor fuel but can also make weapons-grade material at higher levels. Iran strongly denies that it seeks nuclear arms.

The Western-backed demands include halting Iran’s production of 20 percent enriched uranium – higher than the 3.5 percent needed for its lone energy-producing reactor – and shipping the stockpile out of the country. Iran says the 20 percent material is needed at a separate reactor for medical research, but Western officials and allies worry that the higher-enriched fuel could be turned into warhead cores in a matter of months.

Iran has countered with signals that it could eventually suspend its production of the 20 percent enrichment, but with concessions in return. Such concessions could include lifting some of the economic sanctions that have targeted Iran’s oil exports and sharply complicated its links to international banking networks.