Russia is guilty of prolonging the war in Syria and may have committed war crimes by targeting an aid convoy, British Foreign Secretary Boris Johnson said in an interview aired on Sunday.

A 31-truck convoy was attacked on Monday night, killing around 20 civilians. US officials believe Russian aircraft were responsible for the strike, but Moscow has denied involvement.

"(Russia) are guilty of protracting this war and making it far more hideous," Johnson told the BBC's Andrew Marr show.

"When it comes to instances such as the bombing of aid targets in Aleppo, we should be looking at whether or not that targeting is done in the knowledge that those are wholly innocent civilian targets, that is a war crime."