U.S. President Barack Obama on Tuesday warned against rushing to judgment on the use of chemical weapons in Syria, saying he needs "hard, effective evidence" before taking further moves.

Addressing a press conference at the White House to mark 100 days in office in his second term, the president also vowed to take "a spectrum of options" not applied before once he gets the facts.

"What we now have is evidence that chemical weapons have been used inside of Syria, but we don't know how they were used, when they were used, who used them," Obama said, adding "We don't have a chain of custody that establishes what exactly happened."

The White House stated last week that "The U.S. intelligence community assesses with some degree of varying confidence that the Syrian regime has used chemical weapons on a small scale in Syria, specifically the chemical agent sarin," an assertion that had been made by Britain, France and Israel days earlier.

Obama has said time and again that the use of or failure to secure the chemical weapons by the Syrian government will be a " game changer" in his handling of the Syrian conflict, which has dragged on for three years and killed more than 70,000 people.

"That is a game changer because what that portends is potentially even more devastating attacks on civilians and it raises the strong possibility that those chemical weapons can fall into the wrong hands and get disseminated in ways that would threaten U.S. security or the security of our allies," Obama explained.

"By game changer, I mean that we would have to rethink the range of options that are available to us," he said, adding " Obviously, there are options that are available to me that are on the shelf right now, that we have not deployed. And, you know, that's a spectrum of options."

He warned, however, against making hasty decisions.

"When I am making decisions about America's national security and the potential for taking additional action in response to chemical weapon use, I've gotta make sure I've got the facts. That 's what the American people would expect," he said.

"And if we end up rushing to judgment without hard, effective evidence, then we can find ourselves in the position where we can' t mobilize the international community to support what we do," he warned.

"There may be objections even among some people in the region who are sympathetic with the opposition if we take action," he added. "So, you know, it's important for us to do this in a prudent way."

The president said Washington will do all it can to " investigate and establish with some certainty what exactly has happened in Syria, what is happening in Syria."

He pledged that the U.S. will use all the assets and resources at its disposal, and work with the neighboring countries and the United Nations to establish a clear baseline of facts.

The Syrian government is still denying access to an UN investigation team.

Calling the protracted conflict in Syria a "blemish" on the international community, Obama repeated a call for Syrian President Bashar al-Assad to step down to make way for a political transition in his country.

Sixty-two percent of Americans continue to say their country does not have a responsibility to intervene in the fighting in Syria, according to a CBS News/New York Times poll released on Tuesday. Endit