WASHINGTON -- U.S. intelligence has concluded with "varying degrees of confidence" that the Syrian government has twice used chemical weapons in its fierce civil war, the White House and other top administration officials said Thursday.

Officials also said, however, that more definitive proof was needed and the United States was not ready to escalate its involvement in Syria beyond nonlethal aid, despite President Barack Obama's repeated public assertions that Syria's use of chemical weapons would cross a "red line."

The White House disclosed the new intelligence in letters to two senators, and Defense Secretary Chuck Hagel, traveling in Abu Dhabi, also discussed it with reporters. The letters were sent in response to questions from members of Congress who want the administration to arm the rebels or get involved militarily.

"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," the White House said in its letters.

The Syrian civil war has dragged on for more than two years with an estimated 70,000 dead.

advertisement | advertise on newsday

Shortly after the letters were made public, Secretary of State John Kerry said on Capitol Hill that there were two instances of chemical weapons use. It was not immediately clear what quantity might have been used, or when or what casualties might have resulted. Hagel said many of those details were classified.

Sarin is an odorless nerve agent that can be used as a gas or a liquid, poisoning people when they breathe it, absorb it through their skin or eyes, or take it in through food or water.

Lawmakers from both parties called for a U.S. response.

Sen. Dick Durbin of Illinois, a member of the Democratic leadership, said, "something has to be done."

Sen. John McCain (R-Ariz.) said, "I think it's pretty obvious that that red line has been crossed. Now I hope the administration will consider what we have been recommending now for over two years of this bloodletting and massacre and that is to provide a safe area for the opposition to operate, to establish a no-fly zone and provide weapons to people in the resistance who we trust."