ISTANBUL — Iran is prepared to begin enriching uranium beyond the limits set by the nuclear deal it struck with world powers, a top aide to Supreme Leader Ali Khamenei said ahead of a Sunday deadline Iranian officials gave European leaders to offer more concessions to Tehran.

Ali Akbar Velayati, an adviser to Khamenei on international affairs, made the remarks in a video message posted to the supreme leader’s website late Friday. He said the decision to boost uranium enrichment levels was agreed on by “every component of the establishment” and was a reaction to Iran’s misfortunes following a U.S. withdrawal from the pact last year.

Under the agreement, Tehran agreed to curb its nuclear energy activities in exchange for widespread sanctions relief. The United States abandoned the accord and reimposed a near-total embargo on Iran, and European nations have struggled to maintain the economic benefits promised to Tehran under the deal.

Velayati said Iran would reverse its decision to breach elements of the accord if Europe and the United States “go back to fulfilling their commitments.”

Iranian officials say that Tehran’s actions do not violate the deal, which calls on the United States to “make best efforts in good faith” to sustain the accord and to “refrain from re-introducing or re-imposing sanctions.” The deal allows Tehran and other participants to cease performing their commitments if it believes any or all of the other signatories are failing to uphold the deal.

Last week, Iran announced it had bolstered its stockpile of enriched uranium beyond the 300 kg restriction prescribed by the deal. It currently enriches uranium at a rate of 3.67 percent – far below the 90 percent needed to produce a nuclear weapon.

Velayati did not say how much Iran planned to increase enrichment but mentioned a level of 5 percent he said was necessary for electricity generation at the Bushehr power plant, Iran’s only nuclear reactor. Experts say that together the measures would accelerate the breakout time Iran needs to manufacture enough weapons-grade uranium to assemble a nuclear weapon.

The activities at the Bushehr reactor would be part of a “completely peaceful goal of producing electricity and meeting needs in other industries,” Velayati said. However, he warned that Iran would continue to reduce its commitments under the accord if European nations failed to deliver concessions.

Iran’s moves to increase its nuclear energy activities come as tensions have spiked between the United States and Iran in the Persian Gulf region. In recent weeks, a spate of attacks on commercial tankers near the Strait of Hormuz, a key waterway for global oil shipments, raised fears of a wider military confrontation. The United States blamed Iran for the attacks, which Iranian officials deny.

Iran on Saturday denied reports that its forces had seized a British-flagged tanker after it came to a halt in the Persian Gulf, the state-run IRIB news agency reported.

An Iranian commander had threatened to retaliate against U.K. shipping assets in the gulf following the British navy’s seizure of a tanker transporting Iranian oil through the Mediterranean.

An official with U.K. Maritime Trade Operations, which coordinates shipping in the gulf, told Reuters that the tanker was “safe and well” and had stopped as part of a routine procedure to adjust its arrival time at the next port of call.

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