David Miliband, the former UK foreign secretary, has urged Theresa May and other world leaders not to forget about civilians being bombed in Syria at a time when the UN security council has been “driven out of the picture”.

The former Labour politician, who runs the International Rescue Committee, an aid NGO, said the UK as a member of the security council should not be leaving diplomatic activity in Syria to Russia, Iran and Turkey.

He highlighted renewed evidence that civilians in Idlib and elsewhere were being bombed by the regime of Bashar al-Assad, and the ever-growing refugee crisis, as reasons the UK and others needed to renew pressure at the UN.

He told BBC’s Radio 4 Today programme: “If the Syrian government carries on with this tactic, 2.5 million people are in the firing line. It cannot be right that civilians are being bombed to hell without the UN security council playing any role at all. It has been driven off the scene.”

Miliband said the UK “does have a role here as a member of the UN security council, which is to pay attention to the humanitarian situation that exists”.

He said Syria had been a killing field for seven years and there was a risk that the “tactics Assad is using to end this war are going to create an endless war”.

Turkish officials warned last week that the Assad regime’s assault on Idlib, the last Syrian province under rebel control, could spark a “new wave of migration” and add significantly to the refugee crisis in the region.

Tens of thousands of people have fled from the frontlines in Idlib. According to the UN, about 30,000 refugees moved into Idlib’s heavily populated cities and to camps along the Turkish border in the second week of January.

The violence has cast doubt on efforts to negotiate a Syrian peace settlement brokered by Russia, Turkey and Iran. A new deal had designated Idlib and other regions of Syria as “de-escalation zones” in an attempt to stem the violence and create the conditions for talks, but the bombardment is endangering those talks.