The Foreign Office has said tests reveal that the deadly nerve agent sarin has been used during the Syrian civil war as France’s foreign minister said that he is certain that the chemical weapon had been used by the Assad regime.

Laurent Fabius' statement, which follows tests in France on samples smuggled out of Syria by reporters working for the Le Monde newspaper, appears to clarify what several organisations have been suggesting for a number of months: that chemical weapons have been deployed on a number of occasions.

“These tests show the presence of sarin in various samples in our possession,” Laurent Fabius said in a statement. He added that the test results had been passed to the United Nations. “France is certain that sarin gas was used several times in Syria in limited areas,” he said. In an interview with France 2 television, he added: "There is no doubt that it's the regime and its accomplices".

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“It would be unacceptable that those guilty of these crimes remain unpunished,” Fabius added.

Britain's statement was less specific, not attributing the use of sarin to either side of the conflict. The Foreign Office said samples from Syria were tested at a Government laboratory and the presence of sarin was confirmed, though it did not say when or where the samples were obtained.

Britain has evidence suggesting a number of different chemical agents have been used, "sometimes including sarin, sometimes not," said Britain's ambassador to the United Nations, Mark Lyall Grant.

Both France and the UK have pushed to be allowed to arm a number of rebel groups and have used the suggestion that chemical weapons are being utilised by the regime as an argument in their favour. Last week, the European Union lifted its embargo on supplying weapons to Syria, opening up the possibility that both countries could now legally supply certain opposition groups.

Foreign Secretary, William Hague, said that before any such decision was taken, a peace conference, which is planned for later this month, should first be given an opportunity to find a political resolution to the crisis.

President Barack Obama called in April for a “vigorous investigation”, saying the use of such weapons would be a “game changer” if verified.

The French findings come just hours after a UN report into human rights abuses in the bloody civil war. The report suggested that both sides – government troops loyal to President Bashar al-Assad and a number of the various rebel groups – were responsible for using chemical weapons.

The report found "reasonable grounds" to believe that chemical weapons had been used on four occasions in March and April but could not determine which side was behind them.

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“There are reasonable grounds to believe that limited quantities of toxic chemicals were used. It has not been possible, on the evidence available, to determine the precise chemical agents used, their delivery systems or the perpetrator,” Paulo Pinheiro, who chairs the UN commission of inquiry, told a news conference in Geneva.

The brutality of the civil war shows no sign of abating with murder, rape, summary execution and hostage taking, perpetrated by both sides, increasingly commonplace, according to the report published by the United Nations’ Human Rights Council.

In the report, the council warns that without a political solution to the crisis the gross violations of human rights it has documented will continue, and also worsen with the greater proliferation of arms in the country. The European Union, and especially the UK, is considering providing weapons to some rebel groups after an embargo on the sale of munitions to Syria was lifted last week.

Saying that both sides in the now two year long civil war are guilty of war crimes, the UN body says that forces loyal to the government of President Bashar al-Assad, “have committed murder, torture, rape, forcible displacement, enforced disappearance and other inhumane acts. Many of these crimes were perpetrated as part of widespread or systematic attacks against civilian populations and constitute crimes against humanity”.

The myriad of rebel groups – all fighting for Assad’s ouster, but some secularist, others Islamist – have also committed war crimes, according to the report’s findings, but the violations – which include murder, summary execution, kidnapping and pillage – “did not, however, reach the intensity and scale of those committed by Government forces and affiliated militia”.

The UN report covers the period between 15 January and 15 May, during which time the authors say that, “war crimes and crimes against humanity have become a daily reality in Syria where the harrowing accounts of victims have seared themselves on our conscience.”

An estimated 80,000 people – including thousands of civilians – have been killed during the war, which has also driven more than a million overseas to avoid the fighting. Many analysts consider the conflict a proxy for wider tensions in the Middle East, with the Assad regime being supported by Shia Iran and many of the rebel groups receiving assistance from countries such as Qatar and Saudi Arabia.

But outside support for both sides has led to a quagmire which has prevented either side from gaining the necessary military advantage to win the war. At the same time, grave human rights abuses, and a complete gambit of war crimes, have filled the resulting vacuum.

The UN says that only a political settlement can end the conflict. Yet, joint US and Russian plans for a peace conference later this month in Geneva appear to be in trouble. The political group representing the rebels, the Syrian National Coalition, has refused to participate while fighters from the Lebanese militant group Hezbollah continue to fight on behalf of the government, and Assad has publicly stated that he does not believe the summit can bring peace.

No date has yet been set for the conference, but the UN report concludes that only a political solution, based on the conclusions of an earlier Geneva conference held last year, can bring about an end to the war. “The conflict will end only through a comprehensive, inclusive political process. The international community must prioritise a de-escalation of the war and work within the framework of the 2012 Geneva Communiqué,” it says.

The Geneva Communiqué laid out a six point plan to end the fighting, including a cessation of all violence and rapid progress towards a credible political settlement.

France’s concerns appeared to be supported by the Foreign Office which said samples from Syria were tested at a government laboratory and that the presence of sarin had been confirmed. It did not say when or where the samples were obtained.

A spokeswoman said: “The room for doubt continues to diminish. This is extremely concerning. Use of chemical weapons is a war crime. Assad must grant immediate and unrestricted access to the UN investigation team.”

The news came shortly before government forces claimed victory in the strategic town of Qusair.