US President Barack Obama has warned against a rush to judgment on chemical weapons in Syria, but says proof of their use would trigger a "rethink" of his reluctance to use military force.

As critics complain that he let Syria cross a US "red line," Obama said Washington believed chemical weapons had been used in the country's vicious civil war but did not know exactly who had fired them.

At a White House news conference, Obama also appeared to set the criteria for a US military intervention as established proof that President Bashar al-Assad's regime directly ordered the use of chemical weapons.

Obama faced the press amid rising political pressure over reports by US intelligence that Syrian forces used sarin gas against their foes, despite his previous warnings that deploying chemical weapons would be a "game changer."

"I've got to make sure I've got the facts. That's what the American people would expect," he said.

"If I can establish in a way that not only the United States but also the international community feel confident in the use of chemical weapons by the Assad regime, then that is a game changer.

"By game changer, I mean we would have to rethink the range of options that are available to us.

"There are options that are available to me that are on the shelf right now that we have not deployed, and that's a spectrum of options," Obama said, saying he had asked the Pentagon for plans, but did not divulge them.

Possible Pentagon options include a risky operation to secure chemical weapons stocks to ensure they do not get into the hands of radical groups or to destroy them, either by an assault team or air strikes.

Obama's foes are also calling on him to directly send US arms to rebels, a step he has so far been reluctant to take, or to establish no-fly zones or safe havens to protect hundreds of thousands of refugees.

Hours after Obama's news conference, The Washington Post quoted an unnamed senior official as saying he was now ready to send "lethal weaponry" to the Syrian opposition.

But other officials said the administration was considering only equipment with military uses, possibly including items such as goggles, vehicles and armour, and not lethal arms.

And National Security Council spokeswoman Caitlin Hayden added: "we continue to consider all other possible options that would accomplish our objective of hastening a political transition."

But she added no new announcements were imminent.