In this July 10, 2019 photo, the Iranian flag waves outside of the UN building that hosts the International Atomic Energy Agency, IAEA, office inside in Vienna, Austria. (RONALD ZAK / AP)

TEHRAN - Iran has warned the United States to stop "illegal actions" taken against its diplomats on US soil, the spokesman for the Iranian Foreign Affairs Ministry Saeed Khatibzadeh declared on Saturday.

The warning came France’s foreign minister was quoted as saying that Iran is in the process of building up its nuclear weapons capacity and it is urgent that Tehran and Washington return to a 2015 nuclear agreement.

Washington should refrain from taking "illegal action" against Iranian representatives before international organizations such as the United Nations, the World Bank and the International Monetary Fund, that have their siege on US soil, the Iranian Foreign Affairs Ministry spokesman said

"Iran has officially warned the US that it will sue it before the International Court of Justice if it does not stop its illegal actions against Iranian diplomats in international organizations," Khatibzadeh said, as quoted by news agency IRNA.

The spokesman noted an official warning has been delivered to US authorities through the Foreign Interests Section of the Swiss embassy in Tehran.

Washington should refrain from taking "illegal action" against Iranian representatives before international organizations such as the United Nations, the World Bank and the International Monetary Fund, that have their siege on US soil, Khatibzadeh stated.

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For a long time, he carried on, the US government has been creating "illegal constraints" for Iranian diplomats and their families, disrupting their work and that of other countries.

Despite its commitment as host to several international organizations, Khatibzadeh went on to say, Washington has "never been a proper host," and has consistently harassed diplomats, their families, and children coming from countries it has problems with.

Khatibzadeh also commented on the rumors on the possibility of releasing a vessel of Republic of Korea (ROK) recently seized by Iran in the Gulf, saying the statements by people not related to the issue lack any basis.

Khatibzadeh made the remarks in response to a question by some reporters about the rumors of possible release of the ROK ship, as reported by official news agency IRNA.

He stressed that decisions made by the Iran judiciary which is in charge of the issue, are announced only by its spokesman so the statements by people not related to the issue lack any basis, adding that the ROK ship was seized for polluting the Gulf waters upon an Iranian judiciary order.

Also on Saturday, the Iranian President's Chief of Staff Mahmoud Vaezi slammed the latest sanctions imposed by Washington against Iran as a mere "show" and a sign of the "evil nature" of the outgoing US President Donald Trump's administration.

"The recent US sanctions, while nothing more than a show, are another sign of the evil [nature] of Trump's criminal administration - a bankrupt government that even in its final days cannot put aside its hostility towards Iranians," Vaezi wrote on social media, as quoted by Iran's Press TV.

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Vaezi wrote on his note that it is "surprising" that, despite their "deep brazen enmity," US officials "still claim that they stand by the Iranian people."

Iran has been accelerating its breaches of the nuclear deal and earlier this month started pressing ahead with plans to enrich uranium to 20 percent fissile strength at its underground Fordow nuclear plant. That is the level Tehran achieved before striking the deal with world powers to contain its disputed nuclear ambitions.

The Islamic Republic’s breaches of the nuclear agreement since Trump withdrew the United States from it in 2018 and subsequently imposed sanctions on Tehran may complicate efforts by President-elect Joe Biden, who takes office on Jan 20, to rejoin the pact.

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“This has to stop because Iran and - I say this clearly - is in the process of acquiring nuclear (weapons) capacity.”

The agreement’s main aim was to extend the time Iran would need to produce enough fissile material for a nuclear bomb, if it so chose, to at least a year from roughly two to three months. It also lifted international sanctions against Tehran.

Western diplomats have said Iran’s repeated breaches have already reduced the “breakout time” to well below a year.

Iran denies any intent to weaponize its nuclear program.

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Biden has said he will return the United States to the deal if Iran resumes strict compliance with it. Iran says sanctions must be lifted before it reverses its nuclear breaches.

However, Le Drian said that even if both sides were to return to the deal, it would not be enough.

“Tough discussions will be needed over ballistic proliferation and Iran’s destabilization of its neighbors in the region,” Le Drian said.

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Witn Reuters inputs