BORIS Johnson has urged people to demonstrate outside Russia’s embassy over Moscow’s involvement in the Syrian conflict as MPs heard how life in besieged Aleppo had become “hell on earth”.

During a passionate three-hour Commons emergency debate, the foreign secretary said Russia was “in danger of becoming a pariah nation” and made clear the intentional bombing of hospitals was a war crime.

Calling for all such incidents to be fully investigated, he told MPs: “I would remind this House that in recent history war criminals have been successfully prosecuted decades after their offences.”

Mr Johnson also noted how there was “no commensurate horror” among some of the anti-war protest groups at Moscow’s actions.

"I'd certainly like to see demonstrations outside the Russian embassy. Where is the Stop The War Coalition at the moment? Where are they?" he asked.

Earlier, Labour’s Ann Clwyd urged people to speak up for “our common humanity” and said: "So I would therefore call once again on everyone who cares about the plight of Syrian civilians to picket the Russian embassy in London and in capitals around the world from today.

"Two million, three million, four million people; it can be done. It has been done in the past. That should carry on until the bombing campaign stops and all the relevant players are forced to get around the table to end this horrible war," she declared.

Andrew Mitchell, the former international development secretary who secured the debate, explained how five million people within Syria and six million outside its borders were on the move and were often "unprotected, unfed and unhoused".

He again compared Russia's attacks on Aleppo to the Nazi regime's bombing of Guernica during the Spanish civil war.

And the Tory backbencher warned: "The effects of the crisis in Syria on our children and our grandchildren will be every bit as great as the effects of Brexit."

Fellow Conservative Tom Tugendhat, who served in the British Army, argued that militarily there was no reason a no-fly zone could not be enforced.

"The helicopters that are dropping barrel bombs could easily be brought down by rockets based in Turkey, in Lebanon or indeed from our own Type 45s in the Mediterranean," he said.

But Mr Johnson said while he would consult widely on this urged caution, stressing: “We cannot commit to a no-fly zone unless we are prepared to confront and perhaps…shoot down planes and helicopters that violate that zone. We need to think very carefully about the consequences.”

Whitehall sources said establishing a no-fly zone had “not been ruled out” but the issues surrounding them were complex. This is because for them to work effectively Russia would have to be in agreement and, at present, there was “no forum for dialogue” with Moscow.

The political reality is that the process of bringing any resolution to Syria will have to wait until the US elections are over and there is a new president in the White House. Even then, however, it could be many months before a new ceasefire on the ground is established.

During Commons exchanges, Emily Thornberry for Labour described the plight of innocent civilians in Aleppo as “truly a hell on earth; they are trapped, they are impoverished and they are desperately in need of food, clean water and medical care”.

She said both the forces of Russia, the Assad regime and the jihadis stood "equally condemned in the eyes of public opinion and equally guilty of crimes against humanity".

But despite this the shadow foreign secretary insisted it was crucial for Britain to work with Russia to re-establish the ceasefire however difficult it was.

“We have to have that goal in mind. It is the only conceivable solution, the only possible conceivable solution of bringing relief to the people of Aleppo," she added.

The SNP’s Patrick Grady stressed how his party had opposed the UK’s military involvement in Syria and noted: “We have consistently said that what people in Syria need is bread, not bombs. If we have the technology to drop bombs, surely we have the technology to drop or deliver bread and aid.”

His colleague Margaret Ferrier argued that any reasoning behind military involvement had now “completely fallen apart”, that because of airstrikes the UK had become “part of the chaos” and that a revised military strategy was urgently needed in light of the worsening humanitarian crisis.