Britain's most senior diplomat was talking during an emergency debate in the UK Parliament over the crisis engulfing the country, in particular Aleppo.

Russia has been accused of attacking civilians in Aleppo, with the ancient part of the city said to have just weeks left if the bombardment continues.

Mr Johnson told MPs the British government was doing all it could to highlight what was happening, but he believed people could do more to show how angry they were.

He said: "It is the UK, week after week, that is taking the lead, together with our allies... in highlighting what is happening in Syria to a world where, I'm afraid, the wells of outrage are growing exhausted.

"There is no commensurate horror, it seems to me, among those anti-war groups. I would certainly like to see demonstrations outside the Russian embassy.

"It's up to us in the government to show a lead."

He described what was happening in Syria by saying: "In Aleppo... rebel-held districts have come under furious attack from the Assad regime, from Russia, with the help of Iranian-backed militias.

"At this moment the 235,000 inhabitants of eastern Aleppo are under siege, they are isolated... subjected to constant bombardment and prevented from receiving humanitarian aid.

"Every hospital in eastern Aleppo is believed to have been bombed, some more than once. Hospitals have been targeted with such precision that it is difficult to avoid the conclusion that this must be deliberate policy. 

"The house will know that intentionally attacking a hospital amounts to a war crime. It is time for all these incidents to be properly and fully investigated.... to ensure that justice is done."

He added that if Russia continued bombing, it was at risk of becoming a "pariah nation", with Vladimir Putin seeing his ambition of restoring his country's greatness "turning to ashes".

Lt.-Gen. Sergei Rudskoi of the Russian Military General Staff speaks to the media as a map showing fighting around Aleppo is displayed on screen | Photo: PA Wire

Bombing

Further airstrikes were carried out on rebel-held parts of Aleppo on Tuesday, killing at least eight people, according to activists.

A member of the Aleppo local council, Zakaria Amino, said bombs fell in a number of other rebel-held neighbourhoods and the death toll was expected to rise.

Members of eastern Aleppo's White Helmets rescue force said they rescued one boy and two women from rubble but pulled out the bodies of two toddlers from a residential block shattered by an air attack. 

Earlier in the debate, former international development secretary Andrew Mitchell described attacks on Aleppo by the Assad regime and Russia as being like those on civilians in Guernica in 1937 by German Nazi and Italian fascist air forces, during the Spanish Civil War.

Labour's MP for Wirral South, Alison McGovern, broke down during the debate as she paid tribute to the work, on behalf of the Syrian people, of her friend Jo Cox MP, who was murdered in her constituency Batley and Spen in the week's before the referendum.

It came as Vladimir Putin decided not to travel to Paris for an official visit after France said it wanted to use the occasion to discuss the situation in Syria.