The head of the British Armed Forces told his government that the United Kingdom should be prepared to go to war in Syria, according to a report in The Sunday Times.

Gen. Sir David Richards, chief of the defense staff, warned that a military response to Syria’s alleged use of chemical weapons would have to be on a full scale to be effective. He further cautioned that even creating safe zones would risk dragging UK forces into a military confrontation.

The general also argued that imposing a no-fly zone like the one in Bosnia in 1993 would not be effective because of Syria’s air defense.

“Even to set up a humanitarian safe area would be a major military operation without the co-operation of the Syrians,” he told senior defense figures, according to the report.

The British government said it had “limited, but growing” evidence that chemical weapons have been used, and Prime Minister David Cameron told the BBC that it “does look very much like a war crime [is] being committed in our world, at this time, by the Syrian government.”

On Friday, US President Barack Obama made his first comments about the reports of chemical weapons use, calling it a game changer. ”For the Syrian government to utilize chemical weapons on its people crosses a line that will change my calculus and how the United States approaches these issues.”

US officials declared on Thursday that the Syrian government probably had used chemical weapons twice in March, newly provocative acts in the civil war that has killed more than 70,000 people and displaced hundreds of thousands more. The US assessment followed similar conclusions from Britain, France, Israel, and Qatar — key allies eager for a more aggressive response to Syrian conflict.

Last week Tuesday, top IDF analyst Brig. Gen. Itai Brun told an audience at a security conference in Tel Aviv that the army was quite certain that President Bashar Assad deployed chemical weapons against rebel forces in Syria on March 19.

“To the best of our understanding, the regime used lethal chemical weapons,” said the head of the Research and Analysis Division at the IDF Military Intelligence Directorate.

Brun said further that based on the pictures of the victims — the size of their pupils, “and the foam coming out of their mouths” — the army believed that Assad’s troops had used the lethal nerve gas sarin as a weapon.

Mitch Ginsburg and AP contributed to this report.