VIENNA -- Diplomats from Iran and five world powers recommitted Sunday to salvaging a major nuclear deal amid mounting tensions between the West and Tehran since the U.S. withdrew from the accord and reimposed sanctions.

Representatives of Iran, Germany, France, Britain, China, Russia and the European Union met in Vienna to discuss the 2015 agreement that restricts the Iranian nuclear program.

"The atmosphere was constructive, and the discussions were good," Iranian Deputy Foreign Minister Seyed Abbas Araghchi told reporters after the meeting ended.

"I cannot say that we resolved everything" but all the parties are still "determined to save this deal," he added.

Fu Cong, the head of Chinese delegation, said that while there were "some tense moments" during the meeting, "on the whole the atmosphere was very good. Friendly. And it was very professional."

Both diplomats said there was a general agreement to organize a higher-level meeting of foreign ministers soon, but also that preparations for such a summit needed to be done well. A date has not been set.

Iran is pressuring the remaining parties to the deal to offset the sanctions U.S. President Donald Trump reinstated after pulling out. The country recently surpassed the uranium stockpile and enrichment limits set out in the agreement, saying the action could be reversed if the other parties came up with economic incentives.

Experts warn that higher enrichment level and a growing uranium stockpile narrow the one-year window that Iran would need to have enough material to make an atomic bomb, something Iran denies it wants but that the deal prevented.

So far, neither Iran's announcement that it exceeded the amount of low-enriched uranium allowed under the deal nor its revelation it had begun enriching uranium past the 3.67% purity allowed, to 4.5%, are seen as violations likely to prompt the European parties to invoke a dispute resolution mechanism.

Both of Iran's actions were verified by the U.N.'s nuclear watchdog, the International Atomic Energy Agency.

At the Sunday meeting, Fu said, the Europeans urged Iran to come back to full compliance and Iran urged the European Union, France, Britain and Germany to implement their part of the deal.

Fu said all sides expressed strong opposition against the unilateral imposition of sanctions by the U.S., especially the extraterritorial application of the sanctions. They also voiced support for China's efforts to maintain normal trade and oil relations with Iran, Fu added.

In addition to trade with China, Iran is especially keen on the activation of a barter-type system set up by the Europeans that would allow the continent's businesses to trade with Tehran without violating the U.S. sanctions.

Araghchi said the European system was "not functioning yet, but it is in its final stages."

In the meantime, Iran has taken increasingly provocative actions against ships in the Gulf, including seizing a British tanker and downing a U.S. drone. The U.S. has expanded its military presence in the region and fears are growing of a wider conflict.

A Royal Navy warship arrived Sunday in the Gulf to accompany British-flagged ships passing through the Strait of Hormuz. Britain's Ministry of Defence said the HMS Duncan will join the Frigate HMS Montrose in the Gulf to defend freedom of navigation until a diplomatic resolution is found to secure the key waterway again.

Trump pulled the U.S. out of the nuclear deal last year unilaterally, saying he wanted to negotiate a better one.

Under the provisions of the accord, signatories provided Iran with economic sanctions relief in exchange for curbs on the country's nuclear program, but the latest U.S. sanctions have highlighted the inability of the Europeans, as well as Russia and China, to keep up with their commitments.

Iran's recent moves -- which it defends as permissible after the U.S. withdrawal -- are seen as a way to force the others to openly confront the sanctions.

At the same time, Europe is under pressure from the U.S. to abandon the Iran nuclear accord entirely and is also being squeezed by Iran to offset the ever-crippling effects of American economic sanctions.

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Kirsten Grieshaber in Berlin, Amir Vahdat in Tehran, Iran, and Sylvia Hui in London contributed.