The US says it will not be rushed into a verdict over whether Syria has used chemical weapons just because other countries believe evidence supports that conclusion.
Defence Secretary Chuck Hagel said late on Wednesday at the end of a visit to Egypt that included talks about Syria and other regional issues that decisions on such "serious business" could not be decided on a timeline.
"Suspicions are one thing, evidence is another," Hagel said.
"We have to be very careful here before we make any conclusions, draw any conclusions based on real intelligence."
Hagel said he was not questioning other nations' intelligence, but the United States relied on its own intelligence.
The US has warned that any chemical weapons use by Syria would cross a "red line" that would trigger an unspecified response.
Hagel rejected suggestions the US was undermining its credibility by saying it was continuing to assess the issue, as France, Britain and Israel concluded evidence suggested that chemical arms had been used in Syria's conflict.
A top Israeli military intelligence officer said on Tuesday that evidence supported the conclusion Syrian government forces had used chemical weapons, probably sarin, several times against rebels trying to overthrow President Bashar al-Assad.
Syria's information minister said on Wednesday that Damascus would not resort to chemical weapons against its own citizens or even in the event of war with its neighbour Israel.
Hagel said that while he had discussed the Syrian conflict and chemical weapons with Israeli leaders, he had not been given the findings cited by the intelligence officer.
A bipartisan group of US senators said on Wednesday it had sent a letter to President Barack Obama asking him to say whether the Syrian government or elements backed by it had used chemical weapons in the country since the conflict there began in March 2011.
"We believe this question can be answered straightforwardly without compromising any critical intelligence sources and methods, just as our French, British, and Israeli allies have done," the letter said.
The Syrian government and rebels each accused the other of launching a chemical attack near the embattled city of Aleppo last month.
Syrian authorities acknowledged last year having chemical and biological weapons and said they could be used if foreign countries intervened in the conflict.