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 European parties to the deal to offset the sanctions U.S. President Donald Trump reinstated after pulling out. The country recently surpassed the amount of low-enriched uranium it is allowed to stockpile and started enriching uranium past a 3.67% limit permitted, to 4.5%, saying the actions could be reversed if the Europeans came up with incentives that compensated for the impact of the sanctions on the Iranian economy.

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. Araghchi told reporters in Farsi after the meeting that Iran would continue decreasing its commitments until the Europeans meet its demands.

Experts warn that a 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, Iran's exceeding of the agreement's stockpile and uranium enrichment ceilings have been seen as violations likely to prompt the European signatories to invoke a dispute resolution mechanism. Weapons-grade uranium is enriched at a level of 90%.

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

In recent weeks, Iran broke past the limit on its stockpile of low-enriched uranium, but did not say by how much. The nuclear accord has a stockpile limit of 300 kilograms. However, it also permits Iran to enrich uranium and export it, as it has to Russia in past years.

The head of Iran's Atomic Energy Organization said Sunday that the country has enriched 24 tons of uranium since it reached the 2015 nuclear deal with the other countries and the EU.

Atomic Energy Organization head Ali Akbar Salehi was quoted by state TV as saying Iran "did not enrich 300 kilograms of uranium, but enriched 24 (metric) tons of uranium," or what is 24,000 kilograms (nearly 53,000 pounds.)

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 Defense 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.

Iranian President Hassan Rouhani penned an open letter to new British Prime Minister Boris Johnson that was published on the president's website Sunday. Rouhani extended congratulations and said he hoped the diplomatic ties between their countries would be stronger under Johnson's leadership.

Rouhani said he hoped Johnson's "only one visit to Tehran" while serving as U.K. foreign secretary in 2017 and now his tenure as prime minister lead to a "further deepening of bilateral and multilateral relations."

Under the provisions of the 2015 accord, signatories provided Iran with economic sanctions relief in exchange for curbs on the country's nuclear program. Trump withdrew the U.S. and put sanctions on Iran back in place, saying he wanted to negotiate a better agreement.

The U.S. sanctions have had their intended purpose of hurting Iran's economy while highlighting the inability of the Europeans, as well as Russia and China, to keep their commitments.

Meanwhile, the Trump administration is trying to decide whether to risk stoking international tensions even more by ending one of the last remaining components of the 2015 nuclear deal. The U.S. faces a Thursday deadline to decide whether to extend or cancel sanctions waivers to foreign companies working on Iran's civilian nuclear program as permitted under the deal.

Ending the waivers would be the next logical step in the campaign and it's a move favored by Trump's allies in Congress who endorse a tough approach to Iran. But it also would escalate tensions with Iran and with some European allies, and two officials say a divided administration is likely to keep the waivers afloat with temporary extensions. The officials spoke on condition of anonymity to discuss internal deliberations.

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Sometime before Thursday, the administration will have to either cancel or extend waivers that allow European, Russian and Chinese companies to work in Iran's civilian nuclear facilities. The officials familiar with the "civil nuclear cooperation waivers" say a decision in principle has been made to let them expire but that they are likely to be extended for 90 more days to allow companies time to wind down their operations.

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