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The is still pursuing talks on but will work with allies to increase pressure on Iran if diplomacy fails, White House National Security Adviser Jake Sullivan said on Friday.

The talks were close to an agreement until made last-minute demands of the United States, insisting that sanctions imposed on Moscow over its invasion of Ukraine would not hurt its trade with Iran.

USÌýofficials have been cautious in their assessment of efforts to revive the 2015 nuclear accord between Iran and world powers, which would curb Tehran's nuclear programme in exchange for lifting tough sanctions on Iran's economy.

Substantial progress has been made in resolving a number of issues necessary for Washington to come back to the deal "on a compliance-for-compliance basis," Sullivan told reporters aboard Air Force One en route to Poland with President Joe Biden.

"There still are issues left. There still is work to be done," he added. "We are still seeking a diplomatic outcome here that puts Iran's nuclear program back in a box. Of course, if diplomacy doesn't succeed, then we will work very closely with our international partners to increase the pressure on Iran."

Iran Foreign Minister Hossein Amirabdollahian said on Thursday that the vital issue of sanctions relief for Iran was not yet fully resolved.

"If the United States is pragmatic, a nuclear deal can be reached in the short term," he said during a news conference in .

(Reuters)