Several Council members requested the meeting over the latest report from the UN's International Atomic Energy Agency, which said Iran had significantly increased its stockpile of highly enriched uranium to 60 percent purity -- close to the 90 percent needed for an atomic bomb.

US President Donald Trump "has made clear that Iran's nuclear program poses a threat to international peace and security, which the Security Council is charged with protecting," the US mission said after the meeting.

It accused Tehran of continuing to "flagrantly defy" the Council and violate IAEA safeguards.

"The Council must be clear and united in addressing and condemning this brazen behavior," the US statement said, adding that Washington would persist with Trump's "maximum pressure strategy" to keep Tehran from getting a nuclear weapon.

Since his return to the White House in January, the Republican president has called for a new nuclear deal with Iran, but Tehran says no such agreement is possible as long as punishing sanctions remain in place.

Trump said he sent a letter to supreme leader Ayatollah Ali Khamenei urging negotiations -- and warning of possible military action if Iran refuses.

The government in Tehran has received the letter, Iranian media said Wednesday.

Khamenei meanwhile reiterated that his country was not seeking to develop atomic weapons and called US threats "unwise."

He also said Trump's offer of talks was aimed at "deceiving the world's public opinion" by presenting the United States as ready to negotiate and Iran as unwilling.

Back at UN headquarters in New York, Britain's deputy UN ambassador James Kariuki evoked the possibility of activating the so-called "snapback" mechanism -- which would allow for the reimposition of international sanctions on Tehran.

"We are clear that we will take any diplomatic measures to prevent Iran acquiring a nuclear weapon, and that includes the use of snapback if needed," Kariuki told reporters after the Council meeting.

Britain, France and Germany had already sent a letter to the Council in December warning of the "snapback" possibility.

In 2015, Iran reached a landmark deal on its nuclear program with Britain, China, France, Germany, Russia and the United States, which calls for a lessening of sanctions targeting Tehran.

But in 2018, during his first White House term, Trump withdrew from the deal and reimposed heavy sanctions.

China said Wednesday it would host three-way nuclear talks with Russia and Iran later this week.