The head of Syria’s White Helmet volunteers has told Britain’s foreign secretary that the west must do more to protect millions of civilians still in extreme danger, and warned that reconstruction should not begin without justice or accountability.

Raed al-Saleh said Moscow’s record of broken promises meant he had little confidence in a ceasefire brokered between Russia and Turkey in September. The agreement halted a planned advance on Idlib, one of the last parts of Syria still outside the control of President Bashar al-Assad, and currently home to more than 3 million people.

But similar agreements in other parts of Syria have proved a prelude to siege, brutal bombardment and eventual evacuation, and Saleh said he wanted to remind Jeremy Hunt that western support was vital to save lives.

“We would emphasise that we cannot rely on the Russians or trust guarantees provided by them, because we have seen how their commitments were fake in previous de-escalation zones in Homs, in Daraa, in eastern Ghouta,” Saleh said before his meeting with Hunt last week. “So we need more commitment from the western world to protect the Syrian people.”

Syria summit: call for 'lasting ceasefire' at last rebel-held stronghold, Idlib Read more

Western powers, such as Turkey, have strategic, as well as humanitarian, incentives to avoid an all-out assault on Idlib. Previous offensives have ended with mass evacuations, as opposition supporters retreated to other parts of shrinking territory, but Idlib is the last major stronghold. If it is attacked or falls, residents will have to surrender to Assad’s forces or try to cross the border into Turkey.

“A military offensive means a disaster, and not only for Idlib,” Saleh said. “It will have an impact on neighbouring countries, as it would push hundreds of thousands, maybe millions, of people to march to the border trying to escape.”

Turkey already hosts more than 3.5 million Syrians and has closed its border to refugees. But a huge surge of desperate civilians fleeing might pose a serious challenge to that closed-door policy; some among the exodus might also try to push onwards towards Europe.

Saleh hopes that a longer-term ceasefire deal might help to reverse the flow of Syrians abroad. The White Helmets have used the weeks of calm since the September deal to start small-scale reconstruction, clearing rubble to build a park, and restoring power lines. “We believe there is very important work to be done on stabilisation, with the potential to reduce the scale of the refugee crisis, by improving life conditions inside Syria, and maybe inspiring people to return, and paving the way for reconstruction,” said Saleh.

But he warned against western support for larger, national reconstruction programmes, unless there is justice for the dead and jailed. In August, Vladimir Putin urged the international community to start rebuilding Syria so refugees can return, and there have been calls to speed up efforts to broker an end to the war, even if that means leaving Assad in power.

But the fears and frustrations that drove the civil war have only been exacerbated by it, Saleh said. Although Isis atrocities dominated international news for years, Assad and his allies are responsible for the vast majority of civilian deaths.

“There are parties currently who want to try to claim that it’s time now for reconstruction and constitutional reform, as if the crisis had ended, which is not true,” Saleh said, laying out his message for Hunt.

“We want to send a message that there is a need for accountability first in order to build peace. We cannot practically work on reconstruction and constitutional reform if there is no political solution, and there is no transitional justice and accountability mechanisms.”

Play Video 1:31 The Syrian teenager tweeting the horror of life in Ghouta – video

The White Helmets have been hailed for their humanitarian search and rescue work on the frontlines of Syria’s brutal civil war. But Saleh said they are worried about the impact of extensive campaigns of disinformation, which have falsely claimed the group stage rescue efforts and attack scenes, and that they have links to extremist groups.

“Imagine when someone is putting his life in danger to rescue others, then he hears that people are calling them a bad guy or terrorist. That over time may affect the morale of the volunteers.”

“We are very keen to keep the support of ordinary people around the world. The disinformation campaign causes distraction and affects how people view and understand the situation.”

After the meeting on Thursday, Hunt paid tribute to the courage of the White Helmets and said Britain, which helped arrange asylum for hundreds of members trapped by a summer offensive, would continue to support the group.

“The White Helmets risked life and limb to save over 115,000 lives during the Syrian conflict, despite attacks at the hands of the Syrian regime and the Russian military,” Hunt said. “The UK is proud to stand behind them.”