French security services believe the Iranian government was behind a foiled bomb plot that aimed to target an opposition group in Paris in June, a diplomatic source told AFP.

“The head of operations at the intelligence ministry ordered it,” the source said Tuesday on condition of anonymity, referring to an alleged plot to bomb a meeting of the People’s Mujahedeen of Iran (MEK) in a Paris suburb.

Earlier on Wednesday, the French government announced it had frozen assets belonging to two Iranians and Iran’s ministry of security and intelligence in retaliation for the alleged attack plot. The plot had reportedly been thwarted by Israel’s Mossad intelligence agency.

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“This extremely serious act envisaged on our territory could not go without a response,” France’s interior, foreign and economy ministers said in a rare joint statement.

“In taking this decision, France underlines its determination to fight against terrorism in all its forms, particularly on its own territory,” they added.

The foiled attack in June was to have targeted a meeting of some 25,000 Iranian opposition supporters which was also attended by leading US figures, including close allies of US President Donald Trump.

Six people were arrested afterwards in coordinated raids by European police forces, including an Iranian diplomat called Assadollah Asadi, who is believed to be the head of the cell and is set to be extradited from Germany to Belgium for prosecution.

The cell also consisted of two Belgian nationals and an alleged accomplice in France. The Belgian nationals, a husband and wife identified as Amir S. and Nasimeh N., were charged earlier this month with their role in the plot.

The couple, described by Belgian prosecutors as being “of Iranian origin,” carried 500 grams (about a pound) of the volatile explosive TATP along with a detonation device when an elite police squad stopped them in a residential district of Brussels.

Asadi was targeted by France for the six-month asset freeze on Tuesday along with another man named as Saeid Hashemi Moghadam.

On Wednesday, Israel’s Hadashot news reported that intelligence from the Mossad spy agency foiled the bomb plot. According to the report, Israeli agents with their European counterparts led a hunt for the suspects across the continent, leading to the arrests of the suspects.

Iran has denied any involvement, and claimed the opposition group had orchestrated the plot to discredit Iranian President Hassan Rouhani as he embarked on a tour of Europe.

Iranian diplomat arrested in Germany over terror plot: Allegedly instructed an Iranian couple from Belgium to bomb MEK rally. https://t.co/VC3EckhIgb — Björn Stritzel (@bjoernstritzel) July 2, 2018

A statement from Iran’s Foreign Affairs Ministry on Wednesday “forcefully” called for its detained diplomats to be released.

“We deny the accusations and forcefully condemn the Iranian diplomat’s arrest and call for his immediate release,” the statement said.

The counter measures by France could have major diplomatic repercussions at a time when France is working to keep Iran in the 2015 accord to limit its nuclear program amid major tensions in the Middle East.

French President Emmanuel Macron had vowed to visit Tehran early in his term as president, but major differences between the countries on issues such as the wars in Syria and Yemen, as well as the bomb plot, have led to tensions.