The White House said Tuesday that more corroboration was needed that Syrian forces had used chemical weapons, after France said it had proof such arms had been deployed in the war.

President Barack Obama's administration faced new pressure to act on a perceived violation of its "red line" stating President Bashar al-Assad must not use chemical weapons, following new information released in Paris.

French Foreign Minister Laurent Fabius said Tuesday, citing tests carried out by a French laboratory, that sarin was used "several times in a localized manner" but did not give details of where, when or by whom it was used.

White House spokesman Jay Carney did not give a definitive response to the French claim, but said Washington was working with France, Britain and the Syrian opposition to probe the possible use of chemical weapons.

"I would note that the French report that you're citing said that more work needs to be done to establish who is responsible for the use and the amount that was used and more details about the circumstances around it," Carney told a reporter.

"As the president made clear, we need to expand the evidence we have, we need to make it reviewable, we need to have it corroborated before we make any decisions based on the clear violation that use of chemicals would represent by the Syrian regime."

Despite warning Syria that the use of chemical weapons would be a "game changer," Obama has said that he cannot make decisions based on "perceived" violations of US red lines and needs definitive proof.

Obama, who has pulled US troops out of Iraq and will withdraw combat troops from Afghanistan at the end of 2014, has been reluctant to become directly embroiled in the war in Syria, despite rising political pressure.

Carney also lashed the Assad regime Tuesday for refusing to allowing International Committee of the Red Cross (ICRS) officials into the strategic town of Qusayr to aid trapped civilians.

"We are deeply concerned by the continued fighting in Qusayr and condemn the indiscriminate killing of civilians by Assad's forces and his proxies, including Hezbollah fighters," Carney said.

"The regime's siege of Qusayr has created a dire humanitarian situation, with severe shortages of food, water and medicine."

Carney also raised disquiet about the recent spillover of violence into Lebanon, which he said could spark risks of regional instability.

He said Hassan Nasrallah, leader of the Lebanese Shiite militia Hezbollah, was playing a dangerous game by sending fighters to support Assad in Syria.

"Hassan Nasrallah is risking Lebanon's stability and the security of the Lebanese people in order to preserve Bashar al-Assad's rule," Carney said.

"We reaffirm our support for Lebanon's policy of dissociation from the conflict in Syria and urge all parties to avoid actions that will involve the Lebanese people in the conflict."

Developments surrounding chemical weapons in Syria
Paris (AFP) June 04, 2013 - Below is an account of the main developments surrounding accusations of the use of chemical weapons in Syria, after France said Tuesday it has proof sarin gas has been used there.

--2012--

- July 23: The Syrian regime acknowledges for the first time that it has chemical weapons and threatens to use them in case of a Western military intervention, but never against its own people.

- August 20: US President Barack Obama warns Syria of "enormous consequences" if it resorts to chemical weapons, and says the United States would regard any recourse by Damascus to its deadly arsenal as crossing a "red line" which would result in US action.

- December 3: NATO and the United States warn Syria against using its chemical weapons, as a US official says that Syria has begun mixing chemicals that could be used to make sarin gas, a deadly nerve agent.

- December 24: Russian Foreign Minister Sergei Lavrov says it would be "political suicide" for the government of Syrian President Bashar al-Assad if it used chemical weapons against the armed opposition, but that "I do not believe Syria would use chemical weapons."

--2013--

- January 30: the Israeli air force bombards a site of ground-to-air missiles and an adjacent military complex near Damascus suspected of holding chemical agents, with Israel saying it feared the transfer of weapons to the Lebanese Shiite movement Hezbollah, according to a US official.

- March 19: The government and rebel forces accuse each other of using chemical weapons for the first time, but the United States says it has seen "no evidence" of such action.

- March 20: Obama warns the Damascus regime it would be held accountable for any use of chemical weapons against civilians in Syria, which he said would be a "grave and tragic mistake" and a "game-changer".

- April 8: UN Secretary General Ban Ki-moon says the UN inspection team is in Cyprus and ready to deploy. Syria rejects the mission as proposed by Ban.

- April 25: The White House says that Syria has likely used chemical weapons against rebel forces on a "small scale" during the conflict, while emphasising that US spy agencies are still not 100 percent sure.

- May 10: US Secretary of State John Kerry says there is "strong evidence" that the regime has used chemical weapons against rebel forces.

- May 18: Assad denies that his government has used chemical weapons against civilians.

- May 27: French Foreign Minister Laurent Fabius says there are "mounting suspicions" that chemical weapons are being used in Syria.

- May 29: Britain informs the United Nations of "new incidents" of apparent chemical weapons use in Syria, diplomats say.

- June 4: UN investigators say they have "reasonable grounds" to believe that both sides in Syria have used chemical weapons, on four occasions in March and April, adding that "the majority concern their use by government forces".

France says it has proof that Assad's regime is using sarin gas, adding that "all options", including armed intervention, are on the table.

The White House says it needs more evidence.