US President Donald Trump said Iran's Supreme Leader Ayatollah Ali Khamenei should be very careful about what he says after Khamenei harshly criticized the US in a Friday prayers sermon in Tehran.

"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," Trump said in a tweet. "Their economy is crashing, and their people are suffering. He should be very careful with his words!"

In his first Friday prayers sermon for eight years, Khamenei told worshippers chanting "Death to America" that the elite Guards could take their fight beyond Iran's borders. 

"Our enemies ... were happy that they found an excuse to undermine the Guards, the armed forces and our system," Khamenei said, heaping praise on the Guards for protecting Iran and renewing a call for US troops to leave the region.

Khamenei said late General Qassem Soleimani's work of projecting Iran's military influence abroad would continue and the Quds Force he commanded "protects oppressed nations across the region." 

Tension has  ratcheted higher since 2018, when the US withdrew from Tehran's nuclear pact and reimposed sanctions.