The Syrian deputy foreign minister Faisal Mekdad says Obama's decision "shows he and his administration are lost - they don't know what they are going to do."
US Secretary of State John Kerry has said that new evidence has tested positive for use of sarin gas in the Syrian chemical attacks, Reuters reports, citing US broadcaster CNN.
He also voiced confidence that Congress "will do the right thing" on President Obama's request for approval of a unilateral military strike against the Syrian regime.
Groups of demonstrators gathered in various cities in the United States yesterday to protest President Obama's proposed military action in Syria.
In Washington DC, protesters held banners outside the White House urging the president not to start another war.
Meanwhile, in New York protesters chanted "Hands Off Syria" as they marched down Times Square.
US authorities are tightening up domestic security measures ahead of a possible military strike on Syria, according to CNN.
The FBI and the Department of Homeland Security are warning of a higher risk of cyber attacks after months of similar disruptions by hackers known as the Syrian Electronic Army.
One such attack brought down the website of The New York Times in recent days, and authorities say more attacks are likely.
The US Senate is to vote on a resolution authorising military force against Syria no later than week of September 9, the Senate majority leader Harry Reid told Reuters.
Mr Reid said the Senate would hold public hearings on the issue next week with senior Obama administration officials, and would hold classified and unclassified briefings for senators throughout the week.
"I believe the use of military force against Syria is both justified and necessary," Mr Reid said in a statement, saying President Assad had committed "atrocities" against civilians with a chemical weapons attack.
President Obama has sent a draft legislation to Congress formally asking for approval to use military force in Syria to "deter, disrupt, prevent and degrade" the potential for further chemical attacks.
President Obama has formally requested authorisation for military strikes on Syria from Congress, according to Reuters.
The draft bill would authorise Mr Obama to use military force on Syria as necessary to prevent more chemical attacks, the White House said.
The White House has released images of President Barack Obama meeting in the US Government Situation Room with his national security advisers to discuss strategy in Syria.
US Senators John McCain and Lindsey Graham have released a statement supporting President Obama's remarks on Syria.
In it, they said that they believed President Obama was right to say that the Assad regime's use of chemical weapons 'requires a military response by the United States and our friends and allies'.
– Statement from US Senators John McCain and Lindsey Graham
However, we cannot in good conscience support isolated military strikes in Syria that are not part of an overall strategy that can change the momentum on the battlefield, achieve the President's stated goal of Assad's removal from power, and bring an end to this conflict, which is a growing threat to our national security interests.
Anything short of this would be an inadequate response to the crimes against humanity that Assad and his forces are committing.
The statement said anything less would "send the wrong signal to America's friends and allies, the Syrian opposition, the Assad regime, Iran, and the world".
They added that since Obama is now seeking Congressional support for this action, the Congress must act as soon as possible.
A rebel spokesman told NBC News:
President Obama is sending contradictory messages. He promised to help, and now promises delays.
He first said that using chemicals weapons was a red line, which meant there was a green light for killing Syrians with bombs and tanks and guns.
Now the red line was crossed, and crossed again.
President Obama says he wants to act, but is behaving like the British saying he can only act with the will of his people.
If congress votes against a military action, it will mean the American people don't want to help the Syrian people.