Russia has lured aid workers out in the open with "double tap" bombing raids in Syria, Boris Johnson has said.
Blasts send civilians scrambling to escape only for a second round of bombs to be dropped, according to the Foreign Secretary.
The Government has evidence that Russian jets have carried out the attacks, which are "unquestionably a war crime", he said.
Mr Johnson also appeared to take a swipe at former prime minister David Cameron and US president Barack Obama over their failure to deal with the Syrian civil war.
He told The Sun: "One thing I think is certainly a war crime is the double tap procedure that they do.
"They drop one bomb and then they wait for the aid workers to come out, civilian people pulling the injured from the rubble, and then five minutes later they drop another bomb.
"We have evidence. We have good ground to believe that the Russians themselves have been doing that.
"We are trying to document that fully because that is in my view unquestionably a war crime."
Mr Johnson said the West's decision not to intervene against President Bashar Assad in 2013 was "a mistake" that "left the space open for the Russians".
He added: "I think the failure to foresee that was a real error".
Britain and the US are looking at "a range of options" to ramp up pressure on Russia, he said.
The Sun reported that that includes all UK officials boycotting the 2018 World Cup, which Russia is hosting.
Mr Johnson said: "The single most potent weapon we have is shame.
"We are looking at a range of options. There is no point in just mouthing empty rhetoric about these things - you somehow have to make it tell.
"The world's attitude towards Russia has been hardening and I think people now believe that Russia is in danger of becoming a pariah nation.
"If they continue like this they will forfeit any sympathy and any admiration in the world at all, and I think they do care about that.
"In the end, if (Vladimir) Putin's strategy is the greatness and glory of Russia then he risks seeing that turned to ashes as people view his actions with contempt."