A group of tourists cruising along the Zambezi River got the shock of their lives when they found crocodiles eating a human body, a Zimbabwean conservationist said on Friday.

Trevor Lane of the Bhejane Trust said the crocodiles were eating "the bottom half of a human".

The man was suspected to have been an ivory poacher.

"It was a sunset cruise," Lane told a Sapa correspondent.

The tourists, believed to be English, were staying at a camp in the Siansimba area of Zambia in mid-February. They were cruising on the Zambezi, which forms the border between Zambia and Zimbabwe.

The visitors and the guide on their boat saw crocodiles feeding on something in the river. When the boat neared the crocodiles, they saw what was left of a man.

"We strongly suspect it was a poacher," Lane said.

He said that two days earlier, rangers had mounted an ambush in the area to try to catch a group of poachers believed to have crossed from Zambia.

There were 13 poachers altogether. One of them was killed outright. Local intelligence showed that only nine made it back across the river. Three of them have still not been accounted for.

"Maybe he was wounded and tried to swim across the river," Lane said.

The grisly discovery was immediately reported to the authorities in Zambia when the boat docked.

But when officials rushed to the spot where the crocodiles had been seen, they found no remains left.

The Bhejane Trust operates in Zimbabwe's Zambezi and Hwange National Parks.

Crocodile attacks are reported sporadically in Zimbabwe. Three youngsters have been reported killed so far this year, according to local press reports.

The Manica Post reported on Friday that a large crocodile from the Mazonwe River in Burma Valley in eastern Zimbabwe had finally been killed by state rangers earlier in the week.

The newspaper said the crocodile had killed 10 people since mid-2014.