Boris Johnson and his French counterpart, Jean-Marc Ayrault, have urged the international community to “go further” in punishing those responsible for the chemical weapons attack in Syria on 4 April.

In a strongly-worded editorial for the Guardian, the UK and French foreign ministers make clear they would like to see action against the perpetrators of the attacks, once evidence is gathered. They do not mention Johnson’s proposal for punitive sanctions.

“The international community has a moral duty to go further. It is our responsibility to uphold the strict prohibition of chemical weapons. It is our responsibility to ensure that those who conducted such chemical attacks are held accountable. It is crucial for international peace and security,” the pair argue.

They urge the independent Organisation for the Prohibition of Chemical Weapons to press ahead with its investigation into the attack, saying: “We are confident in this process and we fully support it. There will be no impunity.”

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Johnson returned from the G7 summit in Lucca, Italy, last week, without securing formal backing for his proposal of targeted sanctions against senior military figures responsible for the devastating attack on the town of Khan Sheikhun.

Foreign Office sources said Johnson and Ayrault had discussed the sanctions proposals on the sidelines of the summit and expect the measures to be on the agenda for future meetings of European foreign ministers.

Johnson was criticised, including by some in his own party, for raising the prospect of sanctions and then coming back from Lucca empty-handed. The shadow chancellor, John McDonnell, called him a liability for Britain and said he “undermines our credibility”.

But Theresa May defended him on Thursday, saying he had done an “excellent job in bringing together the G7” and delivering a unanimous message of condemnation to Russia.

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In their Guardian article, Johnson and Ayrault set out the latest evidence for the involvement of the regime of the Syrian president, Bashar al-Assad, in the bombing and the fact that deadly sarin nerve agent appears to have been used.

They say: “British scientists have analysed samples from the victims of the attack. These have tested positive for sarin or a sarin-like substance. Our analysis indicates that two Syrian fast jets were in the vicinity of Khan Sheikhun and within range of likely impact sites. We believe that it is only the regime that has the capability to make such an attack. So it is highly likely that attack was carried out by the Assad regime.

“Regime denials should impress no one. Assad said it was the terrorists. But then Assad claims not to have barrel bombs, not to bomb civilians, not to torture, ‘because it would be illogical’. No barrel bombs? Their images are everywhere. No attacks on civilians? Refugee camps in Turkey are replete with their victims. No torture? Amnesty International found that in the Saydnaya prison alone, 13,000 were slaughtered.”

The pair criticise Russia for vetoing a UN security council resolution on Syria and accuse it, together with Iran, of covering up what happened, but they stop short of accusing Russia of complicity in the recent attack.

Donald Trump suggested this week that relations between the US and Russia had hit an “all-time low,” after his secretary of state, Rex Tillerson, travelled to Moscow to urge Vladimir Putin to remove his support for the Assad regime in the wake of the attack.

Trump, who had fought the presidential campaign pledging to stay out of Middle East conflicts, responded to reports of the chemical weapons attack by unleashing a bombing raid on a Syrian airbase.