The British foreign secretary, Jeremy Hunt, has admitted for the first time that Russian support for the Syrian regime means Bashar al-Assad will remain in power for some time.

The UK has been at the forefront of calls for the Syrian president to leave office as part of a transition to a new government, but over the past year British diplomats have acknowledged that Assad would have to be allowed to stand in any UN-supervised democratic elections in Syria.

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In December Donald Trump announced that the remaining 2,000 US troops in Syria would imminently withdraw, leaving Assad in power and a large Iranian military presence intact within Syria.

Many Arab states have also accepted that Assad has survived the seven-year civil war and are preparing to reopen embassies in Damascus.

During a three-day trip to Asia, Hunt told Sky News: “I think you know the British longstanding position is that we won’t have lasting peace in Syria with that regime. But regretfully we do think he’s going to be around for a while and that is because of the support that he’s had from Russia.

“Russia may think that it’s gained a sphere of influence. What we would say to them is: yes – and you’ve also gained a responsibility. If you’re going to be involved in Syria then you need to make sure that there really is peace in Syria. And that means making sure that President Assad does not use chemical weapons.”

In April 2017 Hunt’s predecessor, Boris Johnson, described Assad as a monster that needed decapitating.

Just before Christmas the UN acknowledged it had failed to persuade Russia to agree to form a sufficiently diverse body to prepare a constitution and elections in Syria.

This means UN-supervised elections are delayed indefinitely. The UN special envoy for Syria, Staffan de Mistura, has resigned.

The British acknowledgement of Assad’s continued grip on power underlines doubts that Syrian military leaders will ever be held accountable for the use of chemical weapons in the civil war.

The UK was instrumental in giving the OPCW, the chemical weapons watchdog in The Hague, additional powers to investigate and attribute the use of chemical weapons in Syria.

The UK has claimed Assad’s forces used chemical weapons most recently in Douma in April 2018, an attack that led to joint US, French and British airstrikes. The OPCW has yet to give a formal final verdict on whether Assad’s forces used chemical weapons.

Hunt’s remarks will raise questions about whether European powers will be willing to lift sanctions against Assad or end a moratorium on providing the country with reconstruction aid. The EU has said it will not provide Syria with reconstruction aid until it has assurances that democratic elections will be held.

In Washington, after an outcry over the withdrawal announcement and the resignation of the defence secretary, Jim Mattis, Trump has said he will not allow US troops to leave in a way that endangers the Kurdish forces that have been at the heart of the fight against Islamic State in Northern Syria.

The US had previously set preconditions on a US troop withdrawal including the departure of Iranian militias, but at a briefing on Wednesday Trump said Iran could “do what they want there, frankly,” remarks that left Israel politicians aghast.

Convinced that Isis has been defeated, Trump dismissed the continuing value of Syria to the US, saying: “We are talking about sand and death. We are not talking about vast wealth.”

The UK privately opposed Trump’s decision to withdraw US troops from Syria on the basis that Isis is far from defeated There are also concerns that the Kurds abandoned by the west will release Isis fighters under their guard.

Hunt said he wanted the fate of two British Isis fighters, Alexanda Kotey and El Shafee Elsheikh, to be settled. The two men, who have been stripped of their British citizenship, are being held by Syrian Kurdish forces.