DUBAI (Reuters) - Iran’s supreme leader threw his support behind the elite Revolutionary Guards in a rare sermon on Friday after their belated admission that they had accidentally downed an airliner triggered days of street protests.

In his first Friday prayers sermon for eight years, Ayatollah Ali Khamenei also told worshippers chanting “Death to America” that the elite Guards could take their fight beyond Iran’s borders after the U.S. killing of a top Iranian commander.

U.S. President Donald Trump hit back later on Twitter, writing that Khamenei should be careful what he says.

Khamenei’s address came amid a deepening crisis for Iran as it grapples with unrest at home and rising pressure from abroad.

Tension has steadily ratcheted higher since 2018, when the United States withdrew from Tehran’s nuclear pact with world powers and reimposed sanctions that have hammered the economy.

The standoff erupted into tit-for-tat military strikes this month, when Washington killed top commander Qassem Soleimani in a drone strike on Jan. 3 and Iran launched missile strikes at U.S. targets in Iraq on Jan. 8.

In the tense aftermath, a Ukrainian airliner was shot down by mistake. But it took days for the Guards to admit this and protesters directed their rage at the elite force and the clerical system it was set up to defend.

“Our enemies ... were happy that they found an excuse to undermine the Guards, the armed forces and our system,” Khamenei said in his sermon, heaping praise on the Guards for protecting Iran and renewing a call for U.S. troops to leave the region.

Khamenei said Soleimani’s work of projecting Iran’s military influence abroad would continue and said the Quds Force he commanded “protects oppressed nations across the region.”

He said Quds Force soldiers were “fighters without borders”.

The U.S. State Department’s special representative for Iran, Brian Hook, said in Washington that Iranian threats risked further isolating the country.

But Russia lent Iran some support over the airliner disaster, saying it had been shot down when Tehran was spooked by reports of advanced U.S. stealth fighters in the area.

“I’d like to underline the edginess that always accompanies such situations,” Foreign Minister Sergei Lavrov said.

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‘BITTER TRAGEDY’

Protests led by students erupted in Tehran and other cities for four days over the shooting down of Ukraine International Airlines flight 752, in which all 176 aboard were killed, mostly Iranians or dual nationals.

As the unrest picked up, Trump sent tweets in Farsi and English supporting the protesters, who were chanting “Death to Khamenei” and slogans against the Guards.

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Khamenei said in his sermon: “These American clowns who lie and say they are with the Iranian people should see who the Iranian people are.”

Trump responded on Twitter: “The so-called ‘Supreme Leader’ of Iran, who has not been so Supreme lately, had some nasty things to say about the United States and Europe. Their economy is crashing, and their people are suffering. He should be very careful with his words!”

Khamenei said the shooting down of the plane was a “bitter tragedy that burned through our heart,” but stopped short of offering a direct apology.

“Some tried to use it as an excuse to overshadow the martyrdom of our great commander Soleimani,” he said, urging Iranians to unite and show solidarity by turning out in numbers for the February parliamentary election.

The funeral of Soleimani, long portrayed as a national hero in Iran but seen by the West as a ruthless adversary, had brought huge numbers of Iranian mourners to the streets.

On social media, some Iranians reacted angrily to Khamenei’s sermon and his comments on the plane disaster. “Not a word of apology. Only arrogance,” tweeted Mike Pouraryan.

Online footage during demonstrations had shown protesters being beaten and recordings of gunshots. Riot police were deployed in force on the streets, particularly outside universities, a focus for the unrest.

Iran’s police denied firing at protesters.

Two months earlier, police launched a bloody crackdown on protests that erupted over sharp fuel price hikes, which added to the suffering of Iranians already hurt by U.S. sanctions.

In reaction to Washington’s “maximum pressure” policy, Tehran has gradually scaled back on commitments to the nuclear deal, including lifting limits on its uranium enrichment.

Britain, France and Germany, which have been trying to salvage the pact, have subsequently launched the deal’s dispute mechanism over Iran’s violations, starting a diplomatic process that could lead to reimposing U.N. sanctions.

“These European countries cannot be trusted. Even their negotiations with Iran are full of deceit,” Khamenei said.