UNITED NATIONS - The United States and other Western powers believe Iran's recent ballistic missile launches violate UN resolutions, and have asked the Security Council to address the issue.

Washington, Paris, London and Berlin said as much in a letter dated Monday to the Spanish ambassador Roman Oyarzun Marchesi, who is in charge of the issue within the council.

The same powers last July signed an historic nuclear accord with Tehran to limit its nuclear programme in exchange for the lifting of crippling sanctions.

The letter obtained by AFP says the missile launches were "destabilizing and provocative" and defied a 2015 UN resolution, number 2231. The resolution includes the terms of the nuclear accord.

That agreement lifted most international sanctions imposed on Iran. But it maintained a ban on Iran's launch of ballistic missiles capable of carrying nuclear warheads. The joint letter said the two kinds of missiles fired by Iran in March (Shahab-3 and Qiam-1) "are inherently capable of delivering nuclear weapons."

Iran argues that these missiles are not covered by the UN resolution.

The four signatories of the letter also called on the Security Council to meet in its so-called 2231 format. This is a procedure established to monitor Iran's nuclear activities since the old sanctions committee was disbanded after the nuclear accord was reached.

Diplomats said the meeting was expected to be held Friday. But no firm decisions are expected since Russia refuses to implement new sanctions against its ally Iran.

"The goal is to send a message to the Iranians that we are watching," one diplomat said.

A closed-door meeting of the Security Council on March 14 failed to yield any consensus.

Meanwhile, Iran's supreme leader said Wednesday that missile power was key to the country's future security, slapping down moderates who say the focus should be on diplomacy.

Ayatollah Ali Khamenei, who has the final say in all matters of state in Iran, praised the powerful Revolutionary Guards for their "show of advanced and precise missiles" in recent tests that drew Western criticism.

"In this jungle-like world, if the Islamic republic seeks negotiations, trade and even technology and science, but has no defence power, won't even small countries dare threaten Iran?" Khamenei said in remarks published on his official website.

"Our enemies are constantly enhancing their military and missile capabilities and given this how can we say the age of missiles has passed?"

His comments appeared aimed at ex-president Akbar Hashemi Rafsanjani, a senior leader of the reformist and moderate camp, who last week tweeted: "Tomorrow's world is the world of dialogue not missiles."

They also came a day after the United States, France, Britain and Germany said Iran's recent ballistic missile tests violate UN Security Council resolutions.

The same four countries, along with Russia and China, reached the historic agreement with Iran last year that saw Tehran scale down its nuclear programme in exchange for the lifting of sanctions.

Iran has twice tested ballistic missiles since the July 14 deal, prompting Western condemnation and new US sanctions.

"The enemies of the revolution... use dialogue, economic trade, sanctions, military threats and any other means to further their goals," Khamenei said. "We should be able to confront and defend in all of these fields."

He said those who believe only diplomacy is the key to Iran's future are acting out of "ignorance or treason".