Syrian opposition campaigners have said at least 20 bodies of young men shot by security forces were found in a small waterway running through the contested city of Aleppo.

Sunday's discovery was the largest in a single day of number of bodies lifted from what became known as "the river of martyrs" after 65 bodies washed up in late January.

An average of several bodies a day have been appearing in the river since, several activists in the northern city, which is near Turkey, told Reuters news agency.

Most bodies found so far floated down the River Quwaiq to the opposition-held Bustan al-Qasr neighbourhood after being dumped in an upstream district in central Aleppo under the control of President Bashar al-Assad's forces where several security compounds are located, opposition activists in Aleppo said.

There was no official comment from the government.

State media said the bodies found in January were those of people abducted and killed by al-Nusra Front.

Syrian authorities have banned most independent media, making it difficult to verify reports from inside Syria.

Hands bound, shot in the head

Video footage taken on Sunday, which could not be immediately verified, showed 16 bodies of young men dressed in casual clothes lying on the banks of the small stream.

Some had their hands bound, and many appeared to have been shot in the head or had deep wounds to the neck. Some were gagged. One body was covered with mud and flies.

Louay al-Halabi, an activist in Aleppo, said he was present when the bodies were dug up. He added that bodies had been turning up in Quwaiq when the water level, which is controlled from a government held area, was on the high side.

"I counted 23 bodies today. One man literally had his brains blown out," Halabi said.

Halabi said the men appeared to have been prisoners at security compounds in government-controlled areas taken either dead or alive to a public park in the centre of Aleppo that has been turned into a barracks.

Mohammad Nour, another activist who said he had gone to the site of the bodies, said the men were in their 20s and 30s.

He added that they were a mix of civilians and captured rebel fighters.

"Six men were identified so far. Five of them were detained by air force intelligence last week," he said.

Assad's forces and rebel brigades have been battling in Aleppo, Syria's commercial and industrial hub, since last year.

The two sides have been accused of carrying out summary executions.