A controversial nuclear programme, suspected growing involvement in the Syrian conflict and tightly-controlled presidential elections - Iran is an ever-increasing source of concern for Western powers.
Tehran last week excluded most would-be candidates from its June 14 elections, keeping only those loyal to the all-powerful supreme leader, dashing any hopes that a more moderate President would be voted to power.
As world powers concentrate on rallying support for a June peace conference on Syria, the role of close Damascus ally Tehran in the bloody conflict is coming under increasing scrutiny.
"We see... that day after day Iran's forces are strongly engaged on the side of (Syrian President) Bashar Al-Assad, and this is certainly not the way to advance peace," French foreign minister Laurent Fabius said Sunday.
The issue was likely to come up at a Monday meeting in Paris on the Syria conference - planned for June in Geneva - between Fabius and his US and Russian counterparts.
Iran is accused by Western and Arab countries, which back rebels fighting Assad of supplying weapons and sending military forces to the Syrian military, in a conflict that has claimed some 94,000 lives since 2011.
A top US official said Iranians were working alongside their Shiite Lebanese ally Hezbollah fighters to back Syrian troops battling to retake the rebel stronghold of Qusayr, near the Lebanese border.
Iran has denied this, saying it has never sent military forces to Syria "and will never do so".