TEHRAN, (Reuters) – A senior Iranian cleric warned on Friday that detained British embassy staff would face trial for their alleged role in post-election unrest, and EU countries summoned Iranian envoys to protest against the detentions.

Britain said it was urgently seeking clarification from Iranian authorities over Ayatollah Ahmad Jannati’s comments to worshippers during Friday prayers in Tehran.

“In these developments (the unrest) their embassy here maintained a presence,” Jannati said. “Individuals were arrested and inevitably they will be tried as they have (made) confessions.”

Jannati is a conservative who heads the Guardian Council, a powerful 12-member constitutional watchdog which upheld the official result of the June 12 presidential election — won by hardline President Mahmoud Ahmadinejad but rejected as a fraud by moderate challenger Mirhossein Mousavi.

The post-election unrest has posed a dilemma for Western powers torn between sympathy for protesters and a desire to keep alive chances for dialogue on what they suspect is an Iranian nuclear weapons programme.

Iran denies it is seeking to make bombs, and the incoming head of the U.N.’s nuclear agency said on Friday he did not see any written evidence Iran is seeking nuclear weapons capability. But French President Nicolas Sarkozy said Tehran, which has already been handed three rounds of United Nations sanctions over its failure to stop enriching uranium, could face more international measures over the British embassy detentions.

“France has always wanted to strengthen the sanctions so that the Iranians leaders really understand that the path they have chosen will be a dead end,” he said in Stockholm.

A European Union official in Brussels said members of the 27-member bloc summoned Iranian ambassadors to protest against the detention of the British embassy’s Iranian staff. He said EU states agreed a gradual approach towards Tehran that could in future include visa bans and withdrawal of ambassadors from Iran, depending on how the situation evolved. “The first immediate action is to convey a strong message of protest against the detention of British embassy local staff and to demand their immediate release,” the official said.

Further steps would also be determined by the outcome of the meeting of the Group of Eight major industrialised powers in Italy next week, he said.

Iran said earlier this week that nine Iranian staff at the British embassy were detained for involvement in the mass street protests that erupted after the election.

Most have been released, but British officials say two embassy employees remain in detention. Jannati did not say how many of the staff could face trial.

Britain has denied Iranian accusations that embassy staff were involved in instigating opposition protests after the vote, which defeated presidential candidate Mousavi says was rigged in the incumbent’s favour.

Britain and Iran have already expelled two of each other’s diplomats since the election, which stirred Iran’s most striking display of internal dissent since the 1979 Islamic revolution and strained ties with the West.

In London, Foreign Secretary David Miliband said: “We have noted the remarks by Ayatollah Jannati suggesting that some of our local staff in Iran may face trial. We are urgently seeking clarification from the appropriate Iranian authorities.” He added that he planned to speak to Iranian Foreign Minister Manouchehr Mottaki about the matter. “We are confident that our staff have not engaged in any improper or illegal behaviour. We remain deeply concerned about the two members of our staff who remain in detention in Iran,” he said in a statement.

On Wednesday, Iran’s semi-official Fars News Agency said that one of those detained “had a remarkable role during the recent unrest in managing it behind the scenes”.

Iranian officials deny the election was rigged, saying it was the nation’s “healthiest” since the 1979 Islamic revolution.

The authorities have portrayed the unrest as the work of local subversives and foreign powers, especially Britain.

Jannati reiterated accusations by other senior Iranian figures that the West had plotted a so-called “velvet revolution” to undermine the Islamic Republic’s establishment. “They (the British) had ahead of time … announced that in the election that is scheduled to take place in Iran there might be unrest and turmoil,” Jannati said.

Jannati, who endorsed Ahmadinejad before the election, also hit out at the president’s reformist opponents: “Isn’t their approach an attempt to confront the (Islamic) establishment?”