The measures, which bar dealings linked to the Islamic republic's nuclear and ballistic missile activities, took effect overnight after Western powers triggered the so-called "snapback" mechanism under the 2015 nuclear accord.

"The reactivation of annulled resolutions is legally baseless and unjustifiable... all countries must refrain from recognising this illegal situation," the Iranian foreign ministry said in a statement.

"The Islamic Republic of Iran will firmly defend its national rights and interests, and any action aimed at undermining the rights and interests of its people will face a firm and appropriate response," it added.

The return of the sanctions ends months of tense diplomacy aimed at reviving nuclear talks derailed since June, when Israeli and US forces bombed Iranian nuclear facilities.

Despite the reimposition, Western leaders stressed channels for dialogue remained open.

Kaja Kallas, the European Union's top diplomat, on Sunday said the reimposition of sanctions "must not be the end of diplomacy".

US Secretary of State Marco Rubio urged Iran to "accept direct talks, held in good faith".

He also called on UN member states to "immediately" implement sanctions to "pressure Iran's leaders to do what is right for their nation, and best for the safety of the world".

The British, French and German foreign ministers said in a joint statement they would continue to seek "a new diplomatic solution to ensure Iran never gets a nuclear weapon".

However, Iranian President Masoud Pezeshkian on Sunday rejected any nuclear negotiations that would cause "new problems".

"We have always declared our readiness for a logical, fair and just dialogue based on clear criteria, but we will never accept a negotiation that causes us new problems and issues," Pezeshkian said, according to ISNA news agency.

"The country is ready to face any situation," he noted, adding that "our path is to stand firm, rely on the power of the people, and move with dignity towards a bright future."

- 'No choice' -

Iran had allowed UN inspectors to return to its nuclear sites, but Pezeshkian said in previous remarks that the United States had offered only a short reprieve in return for handing over its whole stockpile of enriched uranium, a proposal he described as unacceptable.

An 11th-hour effort by Iran allies Russia and China to postpone the sanctions until April failed to win enough votes in the Security Council on Friday, leading to the measures taking effect at 3:30 am in Tehran (0000 GMT) on Sunday.

Germany, which triggered the return of sanctions alongside Britain and France, had "no choice" as Iran was not complying with its obligations, Foreign Minister Johann Wadephul said.

"For us, it is imperative: Iran must never acquire a nuclear weapon," he told the UN General Assembly.

"But let me emphasise: we remain open to negotiations on a new agreement. Diplomacy can and should continue."

Iran has long contended that it is not seeking nuclear weapons.

Newspapers in the Islamic republic gave contrasting reactions to the reimposition of the sanctions.

The ultraconservative newspaper Kayhan, which opposes dialogue with the United States, suggested the sanctions would have been imposed even if Iran had engaged in negotiations.

The reformist daily Ham Mihan wrote: "The big question is whether Russia and China will maintain their position".

Russian Foreign Minister Sergei Lavrov on Saturday made clear Moscow would not enforce the sanctions.

- Economic impact -

The sanctions are a "snapback" of measures frozen in 2015 when Iran agreed to major restrictions on its nuclear programme under a deal negotiated by former president Barack Obama.

The United States had already imposed massive sanctions when President Donald Trump withdrew from the deal in his first term.

Iran and the United States had held several rounds of Omani-brokered talks earlier this year before they collapsed in June when first Israel and then the United States attacked Iranian nuclear facilities.

On the ground, Iranians lamented the likely impact of the new sanctions on an already squeezed economy.

"The current (economic) situation was already very difficult, but it's going to get worse," said an Iranian engineer who asked to be identified only by his first name Dariush.

"The impact of the renewed sanctions is already evident: the exchange rate is increasing, and this is leading to higher prices," the 50-year-old said, complaining that the standard of living is "much lower" than it was two or three years ago.

The economic strain was underscored on Sunday when the Iranian rial plunged to a record low against the US dollar on the black market, trading at around 1.12 million per dollar, according to the currency-tracking websites Bonbast and AlanChand.

burs/dcp