US Defence Secretary Chuck Hagel has said American intelligence believes “to some degree of varying confidence” that Syria has used chemical weapons.

It was suspected that sarin gas had been used against rebels, he said while in the United Arab Emirates.

Mr Hagel said the use was “small scale”, and did not give details.

The White House has warned chemical weapons use would be a “red line” for possible intervention, but says this intelligence does not represent proof.

Speaking to reporters in Abu Dhabi on Thursday, Mr Hagel said “our intelligence community does assess, with varying degrees of confidence, that the Syrian regime has used chemical weapons on a small scale in Syria, specifically, the chemical agent sarin”.

“It violates every convention of warfare,” the Pentagon chief added.

‘Red line crossed’

US Secretary of State John Kerry said there had been two instances of chemical weapons use in Syria.

The UK Foreign Office echoed the US claims, saying it had “limited but persuasive information from various sources” of chemical weapons use in Syria.

It is understood that Britain obtained samples from inside Syria which have been tested by the Defence Science and Technology Laboratory at Porton Down, Wiltshire.

“Material from inside Syria tested positive for sarin,” a Foreign Office spokesman said.

Syria is believed to possess large quantities of chemical weapons and there has been heightened concern among the international community in recent months about the safety of the stockpiles.

Although there have been numerous accusations, there has so far not been any confirmation that chemical weapons have been used during Syria’s two-year-old conflict.

US President Barack Obama warned in December that Syrian President Bashar al-Assad would face “consequences” if he used such weapons.

In a letter on Thursday to lawmakers, Miguel Rodriguez, White House director of the office of legislative affairs, said: “Given the stakes involved, and what we have learned from our own recent experiences, intelligence assessments alone are not sufficient – only credible and corroborated facts that provide us with some degree of certainty will guide our decision-making.”

The letter was sent to powerful US senators John McCain and Carl Levin.

In response, Senator McCain told reporters: “It’s pretty obvious that red line has been crossed.”

He recommended arming the opposition, a step the White House has been reluctant to take. He also urged taking steps to ensure that Syria’s chemical weapons did not fall into the wrong hands.

“It does not mean boots on the ground,” the Arizona senator added.

‘No-fly zone’

Democrat Dianne Feinstein, chair of the Senate Intelligence Committee, called for international action to help secure Syria’s stockpile of chemical arms.

Robert Menendez, the Democratic Chairman of the Senate Foreign Relations Committee, said options included “an internationally recognised no-fly zone, providing lethal assistance to vetted opposition forces, and sanctioning the transfer of arms to the regime”.

White House officials said the US would consult with allies and seek more evidence to confirm their intelligence.

On Tuesday, a senior Israeli military official accused Syrian forces of having used the nerve agent sarin against rebels several times.

Speaking at a security conference in Jerusalem, Brigadier General Itai Brun cited photographs of victims foaming at the mouth and with constricted pupils and other unspecified symptoms.

Syria’s government and rebels have accused each other of using chemical weapons. A UN team is trying to enter Syria to investigate.

According to the UN, at least 70,000 people have been killed in the Syrian conflict.

Analysis Mark Mardell BBC North America editor

This is typical President Obama and it will infuriate his critics and reassure his friends in equal measure.

The letter to senators and a White House briefing strongly suggest that chemical weapons have been used in Syria, and they’ve confirmed that would cross the president’s red lines. But they don’t feel the evidence is quite strong enough.

They add that they need solid facts before taking action, adding in a sidelong and pointed reference to the Iraq war that certainty is necessary “given the stakes involved, and what we have learned from our own recent experience”.

Although President Obama has said the use of chemical weapons would be “a game changer”, we don’t know what sort of game it would change into – it could be anything from war to a small scale operation to secure the weapons.

It is obvious Obama wants to avoid military action in Syria – he sees it as too complex and too difficult with no clear objectives. But it looks as if some action will be taken. It is not impossible to back away from action, but it is hard – unless of course the White House decides the facts are wrong.

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