The UN on Monday expressed concern over reports that the United States is deploying a carrier strike group to the Middle East and expanding its military presence in the region.

Asked about the reports, UN spokesperson Stephane Dujarric told a news conference: "We obviously are seeing these reports of heightened military tensions, which are very concerning indeed."

Addressing reports of the alleged use of force by Iranian security forces against protesters, Dujarric said: "Let me just say, we've seen various reports of an extremely high number of people killed by the security forces, of protesters being killed by the security forces."

"The numbers vary, but even without having an exact number, it's clear that the situation on the ground is just horrendous in terms of these killings," he added.

Dujarric said the UN is also alarmed by the broader regional implications of recent developments, stating: "At the same time, we are very concerned about increased military activities and rhetoric, which we think goes counter to the stability of the region."

Tens of thousands have rallied across Iranian cities over economic grievances and the depreciation of the rial, prompting US President Donald Trump to warn of a US "rescue" if protesters are killed, saying forces are "locked and loaded."

Trump has said he is weighing a "tough response," threatening action if mass executions proceed. He later softened his rhetoric, suggesting Tehran had stopped killing protesters following his warning.

Iranian officials have warned that any US attack would trigger a "swift and comprehensive" response. (AA)