WASHINGTON — The White House said Thursday that it believes the Syrian government has used chemical weapons in its civil war, an assessment that could test President Obama’s repeated warnings that such an attack could precipitate American intervention in Syria.

The White House, in a letter to Congressional leaders, said the nation’s intelligence agencies assessed “with varying degrees of confidence” that the government of President Bashar al-Assad had used the chemical agent sarin on a small scale.

But it said more conclusive evidence was needed before Mr. Obama would take action, referring obliquely to both the Bush administration’s use of faulty intelligence in the march to war in Iraq and the ramifications of any decision to enter another conflict in the Middle East.

Senator Dianne Feinstein, Democrat of California, who is chairwoman of the Senate Intelligence Committee, said the agencies actually expressed more certainty about the use of these weapons than the White House indicated in its letter. She said Thursday that they voiced medium to high confidence in their assessment, which officials said was based on the testing of soil samples and blood drawn from people who had been wounded.