These are serious times. Following the missile attacks on Syria, now is the moment for a powerful push for peace. Boris Johnson’s blithe acceptance on Sunday that the conflict will now continue on its current course and that peace negotiations would be an “extra” is an unconscionable abdication of responsibility and morality.

Already this devastating conflict has cost more than 500,000 lives and led to 5 million refugees being forced to flee Syria, and 6 million internally displaced. We must put negotiations for a political settlement centre stage, and not slip into a new cycle of military reaction and counter-reaction.

Protracted external military intervention in Syria – from funding and arms supplies to bombing and boots on the ground – has not helped in the slightest. Syria has become the theatre for military action by regional and international powers – the United States, Britain, Russia, France, Turkey, Iran, Saudi Arabia, Israel, Qatar and the United Arab Emirates among them.

Saturday’s attack on sites thought to be linked to Syria’s chemical weapons capability was both wrong and misconceived. It was either purely symbolic – a demolition of what appear to be empty buildings, already shown to be entirely ineffective as a deterrent – or it was the precursor to wider military action. That would risk a reckless escalation of the war and death toll, and the danger of direct confrontation between the US and Russia. Neither possibility offers an end to the war and suffering, or any prospect of saving lives – rather the opposite. The intensification of military action will simply lead to more deaths and more refugees.

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There can be no question of turning a blind eye to the use of chemical weapons. Their deployment constitutes a crime, and those responsible must be held to account. The Assad government was supposed to have given up its chemical munitions stocks (though not chlorine) under the UN-backed agreement of 2013, and hundreds of tonnes were destroyed under the supervision of the Organisation for the Prohibition of Chemical Weapons, Russia and the US.

Contrary to what is claimed, UN security council agreement was secured then, and again in 2015 and 2016, for an independent UN chemical weapons inspections regime. That can and must be re-established, as both sides in the security council are now proposing.

Inspectors must be given full access to collect evidence, as well as additional powers. Russia must be held to its 2013 commitments, and pressure be exerted on the Assad government to cooperate with investigations into the outrage at Douma.

It is essential to insist on legality and on a UN sanction for any further military action

The same applies to the armed opposition groups, some Saudi- or western-backed, which have also been implicated in the use of chemical weapons. Pressure can also be applied on those found responsible through sanctions, embargoes and, if necessary, through the international criminal court.

Full accountability will depend on an end to the conflict. But there is plenty that can be done now, without adding fuel to the Syrian fire. There are those who are sceptical of multilateral diplomacy. But it is essential to insist on legality and on a UN sanction for any further military action. We can’t accept that a “new cold war” is unavoidable, as the UN secretary general António Guterres has warned. A shift from the rhetoric of endless confrontation with Russia could also help lower the temperature and make a UN consensus for multilateral action to end Syria’s agony more likely.

The military action at the weekend was legally questionable. The government’s own justification, which relies heavily on the strongly contested doctrine of humanitarian intervention, does not even meet its own tests. Without UN authority it was again a matter of the US and British governments arrogating to themselves an authority to act unilaterally which they do not possess.

The fact that the prime minister ordered the attacks without seeking authorisation from parliament only underlines the weakness of a government that was in reality simply waiting for authorisation from a bellicose and unstable US president. That’s why we are pressing for parliament to have the final say on planned military action in future in a new war powers act.

Further military action would be reckless. Even more than was the case in the disastrous interventions in Iraq, Libya and Afghanistan, the continuing war in Syria is fraught with the danger of a wider conflict, starting with Russia and dragging in Turkey, Iran, Israel and others.

Nor is there any political plan on offer. Libya offers the most recent, and calamitous, example of a military operation launched with no thought to the political aftermath. Meanwhile the UK-backed Saudi bombing campaign in Yemen is a humanitarian disaster.

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The British government needs to act as a restraining influence in this crisis, not a camp follower. It is welcome that the UN security council will now be discussing both a new weapons inspections regime and a revival of the stalled peace talks. Such discussions need to be conducted with the aim of agreement, not big power point-scoring.

We have to remove the scourge of chemical weapons but also use our influence to end the still greater scourge of the Syrian war. A diplomatic solution that will allow for the country to be rebuilt, for refugees to be able to return home and for an inclusive political settlement that allows the Syrian people to decide their own future could not be more urgent.

All this, and not a fresh bombing campaign, is what the British people want from their government. Now is the moment for moral and political leadership, not kneejerk military responses.

• Jeremy Corbyn is the leader of the Labour party