WASHINGTON, United States -- The United States said Thursday that "a possible deal" on a new Iranian nuclear accord is close but several sticking points have prevented an agreement and time is running out.

Negotiators meeting in Vienna to try and salvage the 2015 nuclear deal, which is meant to prevent Tehran from acquiring an atomic bomb, have made "significant progress," State Department deputy spokeswoman Jalina Porter told reporters, echoing other nations in recent weeks.

"We are close to a possible deal, but a number of difficult issues still remain unsolved," she said.

"We will not have a deal unless we resolve quickly the remaining issues," she added.

However, "if Iran shows seriousness, we can and should reach an understanding of mutual return to full implementation of the JCPOA within days," she said, using the acronym for the 2015 accord.

Enrique Mora, the European Union's coordinator for the talks, also said they were in the "final stages."

"Some relevant issues are still open and success is never guaranteed," he tweeted, adding "we are definitely not there yet."

The so-called Joint Comprehensive Plan of Action, signed in 2015, secured sanctions relief for Iran in return for strict curbs on its nuclear program.

The agreement was between Iran on one side and Germany, China, the United States, France, Britain and Russia on the other.

The agreement unraveled when former US president Donald Trump withdrew from it, with Israeli encouragement, in 2018.

Iran's stockpile of enriched uranium has now reached more than 15 times the limit set out in the 2015 accord, the UN's IAEA nuclear watchdog said Thursday.

The coming days are seen as critical by the West, which believes that the agreement could soon be irrelevant at the rate Iran is making nuclear advances.

Several observers believe that the West could leave the negotiating table and chalk the deal up to a failure if a compromise is not reached by this weekend.

Among the problem points, Tehran is calling for the closure of the IAEA's investigation into the presence of nuclear material at undeclared sites.

IAEA Director General Rafael Gross, who has said the agency would "never abandon" its attempts to get Iran to clarify the previous presence of nuclear material at the sites, will travel to Iran on Saturday to meet with officials from the country.