At least 56 people were killed in a stampede as tens of thousands of mourners packed streets for the funeral of a slain Iranian military commander in his hometown, forcing his burial to be delayed by several hours, state media said.

General Qassem Soleimani's burial began in the early evening in the southeastern Iranian city of Kerman, four days after his killing in a US drone strike in Iraq.

The attack has plunged the region into a new crisis and raised fears of broader conflict.

"A few minutes ago his body was transferred to the martyrs section of Kerman cemetery," the semi-official news agency ISNA reported, adding that General Soleimani's interment had begun.

General Soleimani, who commanded the elite Quds Force, was responsible for building up Iran's network of proxy armies across the Middle East.

He was a pivotal figure in orchestrating Iran's long-standing campaign to drive US forces out of Iraq.

A senior Iranian official said Iran was considering several scenarios to avenge his death.

Other senior figures have said the Islamic Republic would match the scale of the killing when it responds, but that it would choose the time and place.

The stampede broke out amid the crush of mourners, killing 56 people, state television said, raising the toll from 50 previously.

More than 210 people were injured, an emergency services official told the semi-official Fars news agency.

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"Today because of the heavy congestion of the crowd unfortunately a number of our fellow citizens who were mourning were injured and a number were killed," emergency medical services chief Pirhossein Kolivand told state television.

General Soleimani was a national hero to many Iranians, whether supporters of the clerical leadership or not.

However he was but viewed as a dangerous villain by Western governments opposed to Iran's arc of influence running across the Levant and into the Gulf region.

Iran's opponents say its proxies have fuelled conflicts, killing and displacing people in Iraq, Syria and beyond.

Iran says any operations abroad are at the request of governments and that it offers "advisory support".

General Soleimani's body had been taken to Iraqi and Iranian cities before arriving in Kerman for burial.

In each place, huge numbers of people filled thoroughfares, chanting "Death to America" and weeping with emotion.

Supreme Leader Ayatollah Ali Khamenei wept as he led prayers in Iran.

The US defense secretary has denied reports that the United States was preparing to withdraw troops from Iraq, where Iran has vied with the US for the upper hand since the 2003 US-led invasion.

About 5,000 US troops remain in Iraq, where there has been a US military presence since Saddam Hussein's fall.

Some members of the 29-nation NATO alliance said they were moving some of their personnel that have been training Iraqi security forces out of the country as a precaution amid fears of a regional conflagration.