If US President Donald Trump has wasted no time in offering vocal support to Iran's anti-government protesters, Europe's governments are treading a much more delicate diplomatic line.

European leaders have largely stuck to bland expressions of concern as the demonstrations have spiralled into unrest that has left at least 21 people dead -- far from Trump's outspoken tweets against a "brutal and corrupt" regime.

Trump, who considers the Islamic Republic his enemy number one in the Middle East, on Wednesday promised unspecified support for Iranians trying to "take back" their government -- ignoring warnings that his intervention could backfire.In contrast, the European Union -- which has been normalising ties with Tehran since the nuclear deal sealed in 2015 -- issued a carefully worded statement mourning the loss of life and calling for "all concerned to refrain from violence".

Berlin called for the regime to respond to the protests "through dialogue", urging Tehran to "respect freedom of assembly and expression".

French President Emmanuel Macron, a key defender of the historic Iranian nuclear deal in the face of Trump's opposition, telephoned his Iranian counterpart Hassan Rouhani calling for "restraint".

A visit by French Foreign Minister Jean-Yves Le Drian to Tehran was due in the coming days but has been put back due to the delicacy of the situation.

Britain's reaction, meanwhile, has been limited to a call for "meaningful debate" over the protesters' complaints, with Foreign Secretary Boris Johnson saying: "The UK is watching events in Iran closely."

Protests over high living costs and other economic problems started on December 28 before turning against the regime as a whole.

But on Wednesday, as tens of thousands of regime supporters rallied, analysts said Europe was banking on the anti-government protests coming to little."The European position is certainly less advantageous in terms of public opinion, but it is wiser," said Francois Nicoullaud, a former French ambassador to Iran.

"What is happening in Iran is an expression of deep suffering but without a structure or programme, its chances of success are very limited," he added.

"In a serious clash with the regime, it would be crushed."The EU regards the hard-won nuclear deal as the best chance of stopping Tehran from getting its hands on a nuclear bomb -- unlike Trump, who has left the accord's fate up to Congress.

Some have warned his rhetoric on the protests could backfire by lending ammunition to Iranian hardliners looking for evidence that the West is out to get them.

"It would be unwise for the West, and especially for President Trump, to egg on the protesters, as this could rally the hardliners," Britain's Times newspaper wrote on Monday.

"Better to wait and watch a flawed regime unravel in its own contradictions."

Thierry Coville, an Iran specialist at French think-tank Iris, said Europe's business interests were at play, as well as concerns that Tehran should be handled carefully due to its involvement in Syria, Yemen and beyond.

"Before the economic sanctions against Iran, the EU was this country's most important commercial partner," he said.

"And on a diplomatic level, Iran is a key player in multiple regional crises. All this contributes to the idea in Europe that we need to maintain normalised relations with Tehran," he added.

"The European reaction must be seen in this context."

In his view, he added, the best way to support Iranian civil society activists is "to try to improve the economy -- not to throw oil on the fire."