Washington DC - President Donald Trump expressed outrage on Tuesday at the mounting number of executions in Iran of people allegedly involved in mass protests earlier this year before the Middle East war began.

"The failing Islamic Republic of Iran is not paying large segments of their military, while at the same time killing protesters, even when they are not protesting, at levels not seen before," he wrote on Truth Social.

"It is a humanitarian crisis of epic proportions, and must be stopped, NOW," he added.

It was not immediately clear what new information had spurred the early morning post, but there has been a steady increase in executions since the outbreak of the war on February 28.

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The joint US-Israeli strikes which kicked off the war came weeks after the violent quelling of mass protests in Iran.

Rallies initially began in late December over outrage at the high cost of living, but rapidly evolved into widespread anti-government demonstrations that peaked on January 8 and 9.

Iranian authorities reported more than 3,000 deaths, but attributed the violence to "terrorist acts" orchestrated by the US and Israel.

Rights groups based abroad recorded a far higher toll and said the security forces had fired on demonstrators.

Earlier this month, more than 30 countries signed a joint letter condemning "in the strongest terms the ongoing executions of protesters and use of the death penalty" by Iran.

The US, where capital punishment is still legal, did not sign the letter.

Since then, at least three more people have been executed in Iran on charges related to the protests.