(CNN) -- The United States has evidence that the chemical weapon sarin has been used in Syria on a small scale, Defense Secretary Chuck Hagel said Thursday.

But numerous questions remain about the origins of the chemicals and what impact their apparent use could have on the ongoing Syrian civil war and international involvement in it.

When asked if the intelligence community's conclusion pushed the situation across President Barack Obama's "red line" that could potentially trigger more U.S. involvement in the Syrian civil war, Hagel said U.S. officials are still assessing the situation and need all the facts.

The U.S intelligence community has assessed "with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," according to a White House letter to U.S. Sens. Carl Levin and John McCain.

The letter cautions that "the chain of custody is not clear, so we cannot confirm how the exposure occurred and under what conditions."

The White House is "pressing for a comprehensive United Nations investigation that can credibly evaluate the evidence and establish what took place," according to the letter.

Hagel said that U.S. officials have not been able to confirm the origins of the sarin, but that they believe it originated with the regime of Syrian President Bashar al-Assad, which has been battling a rebellion for more than two years.

The announcement comes a few days after an Israeli intelligence official said Damascus was using weapons banned under international law against its own people in the country's civil war.

Syria has said rebels have used chemical weapons.