Boris Johnson made a last-ditch plea to Russia and Iran to "abandon their puppet" Bashar al-Assad, as the Syrian leader's forces overrun the besieged city of Aleppo.
The foreign secretary told MPs that the UK government are "doing everything we can" amid reports from human rights groups that civilians are being targeted.
"I hope that Russia will see sense and join with us to secure the transition away from Assad, which is the only hope for a peaceful Syria," he said.
"It's up to them – it's up to the Russians, it's up to Iran. They have the future of Syria in their hands. This is one of the darkest hours in Aleppo's four millennia of recorded history."
Johnson also explained how so-called "barrel bombs" were being dropped by regime forces.
"Imagine a metal drum filled with petrol and explosives, laced with nails and jagged shards of metal," he said.
"These objects are loaded on board helicopters, which then hover over civilian areas, the men on the helicopters simply light the fuses of the barrels before rolling them out of the door, leaving them to fall on the ground where they shred and incinerate any human being within range
"There is no guidance system or targeting. Barrel bombs have no military purpose. Their sole purpose is to murder civilians."
The comments came amid an emergency debate in the House of Commons over the Syrian civil war. The debate was granted by Speaker John Bercow after a request from former International Development Secretary Andrew Mitchell and Labour MP Alison McGovern.
Labour leader Jeremy Corbyn has written to Theresa May calling for a concerted effort to achieve an UN-led ceasefire and UN-brokered humanitarian corridors in Syria.
"Jeremy has repeatedly condemned the Russian military intervention and bombing campaign in Syria and called for an independent investigation of evidence of war crimes," a spokesman for Corbyn said.
"Labour has called for urgent talks to achieve a negotiated political settlement involving the main parties to the conflict, along with the regional and international intervening powers – and he has written to the prime minister today calling for a concerted effort to achieve a UN-led ceasefire and UN-brokered humanitarian corridors."
The intervention comes after human rights campaigner Peter Tatchell interrupted Corbyn's speech on Human Rights Day.
Tatchell told IBTimes UK that he wanted Corbyn and Labour to press for a vote in the Commons to back humanitarian air drops on Syria.
"Neither Labour or Jeremy are actively campaigning for any of these initiatives, but during the protest I asked Corbyn to press for a parliamentary vote to mandate UK aid drops of food and medicine to besieged civilians," he said.
"Amid the confusion he didn't give any commitment to push for such a vote. He is the leader of the opposition and should be holding the government to account for its inaction.
"Jeremy should be demanding that [Prime Minister] Theresa May and [Foreign Secretary] Boris Johnson agree to allow a vote in parliament on air drop of humanitarian aid."
The UK government's stance is to seek a "political solution" between Assad and opposition groups to end the Syrian civil war.