VIENNA, Austria — British Foreign Secretary William Hague said Sunday that there had been no “decisive breakthrough” in nuclear talks with Iran in Vienna involving foreign ministers.

“We haven’t made the decisive breakthrough … There are very significant gaps, particularly on that issue (uranium enrichment). There is a huge gap (on that issue),” Hague told reporters.

It was an opinion echoed by US Secretary of State John Kerry.

“We have some very significant gaps,” he said on his arrival.

China urged world powers and Iran “to show flexibility” in talks over the Islamic republic’s nuclear program.

“We urge all parties to show flexibility and political will to reach a comprehensive agreement,” Chinese deputy foreign minister Li Baodong told reporters in Vienna, where the talks are being held.

“On practically all the important issues differences persist and we have not been able to narrow them,” one of Iran’s top negotiators, Abbas Araqchi, told Al-Alam television.

The unbridged positions threaten to prevent the much sought-after, historic deal being struck by its July 20 deadline, when a six-month interim accord with Iran runs out.

If no agreement is reached by Sunday next week, both sides can decide to extend the interim pact for some weeks or months to keep talking.

Lower-ranking officials represented both Russia and China, possibly reflecting their view even before Sunday that talks past July 20 are unavoidable.

But Iranian Deputy Foreign Minister Abbas Araghchi suggested any extension would be relatively short, saying “there is not much willingness” by either side to go a full six months. He, too, earlier spoke of “huge and deep differences.”

The core sticking point of Iran’s uranium enrichment — an activity that can produce fuel for the country’s sole nuclear plant or, if further enriched, the matter for an atomic bomb — has run up against declarations by Ayatollah Ali Khamenei, Iran’s supreme leader who sets the parameters for his country’s negotiators.

The UN Security Council and the International Atomic Energy Agency (IAEA) have long been concerned about Iran’s outsized atomic activities and secret construction of two uranium enrichment sites, one of which is heavily fortified under a mountain. The Security Council has passed six resolutions demanding Iran cease all enrichment, which Tehran has defied.

Despite Iranian insistence that the enrichment is exclusively for energy production and medical purposes, the P5+1 fears Tehran’s program was aimed at attaining nuclear weapon capability.

The talks are designed to limit Iran’s atomic activities so that they can only be used for civilian ends, and that any “breakout” point is pushed back years. Washington wants Iran’s uranium enrichment limited for at least 10 years, a senior US official said Saturday.

The Associated Press contributed to this report.