© Chip Somodevilla/Getty ImagesPresident Obama and his wife Michelle observe a minute's silence on the south lawn of the White House yesterday for the victims of 9/11.
Responding to the plan for an offensive Russia warned that air strikes against militants in Syria would be a "gross violation" of international law.
A Russian foreign ministry spokesman said such action, without the backing of the UN, would be "an act of aggression".
The reaction came as US Secretary of State John Kerry met Arab leaders in Saudi Arabia to try to build a coalition against Isil.
Even on Capitol Hill in Washington regardless of the fact that it was the 13th anniversary of the 9/11 World Trade Centre attacks there was no show of uniformity.
Comment: Isn't it interesting that they are using the anniversary of 9/11 attacks to announce the bombing of Syria.
House of Representatives Speaker John Boehner said that many Republican lawmakers doubt that Mr Obama's plan to destroy the Isil group is strong enough to accomplish that mission.
Mr Boehner, the top Republican in Congress, told a news conference that while Congress stands ready to support the president's efforts, he heard a lot of concerns expressed by Republicans during a meeting on yesterday.
"I'm not sure that we're doing all that we can do to defeat this terrorist threat. And if our goal is to eliminate Isil, there's a lot of doubt whether the plan that was outlined by the president last night is enough to accomplish that mission," he said, using an acronym for the group
He said he supports Mr Obama's request for authority to arm and train moderate Syrian rebels, and believes that they are "well-vetted" by U.S. intelligence services.
Comment: Obama's plan to fight ISIS? Create another ISIS. Arming, funding and training these "moderate" rebels is how ISIS was created. Now the U.S. is planning to do more of the same, which just shows that they have no intent on fighting ISIS.
Isil controls large swathes of Syria and Iraq after a rapid military advance.
In the speech outlining his strategy, Mr Obama said any group that threatened America would "find no safe haven".
He also announced that 475 US military personnel would be sent to Iraq but said they would not have a combat role.
Comment: Right, they'll just train the "moderate" mercenaries.
The statement brought the strongest reaction from Russia, which has been an ally of embattled Syrian President Bashar al-Assad.
"The US president has spoken directly about the possibility of strikes by the US armed forces against Isil (IS) positions in Syria without the consent of the legitimate government," ministry spokesman Alexander Lukashevich was quoted as saying.
"This step, in the absence of a UN Security Council decision, would be an act of aggression, a gross violation of international law."
Syria also repeated its warning that the US had to co-ordinate with the Syrian government before launching air strikes on its territory.
"Any action of any kind without the consent of the Syrian government would be an attack on Syria," National Reconciliation Minister Ali Haidar said on Thursday.
Last month, Syria offered to help the US fight Islamic State. The US has launched more than 150 air strikes against the group in Iraq and has provided arms to Iraqi and Kurdish forces fighting against IS.
Yesterday was the sixth time that President Barack Obama has come before the American people to reckon with the legacy of the Sept. 11 terrorist attacks on the anniversary of that awful day.
Each time, he has tailored his message to the moment, at different points stressing themes of service, resilience, tolerance, reconciliation.
Comment: Saying one thing, doing the complete opposite, that's Obama.
Hard to see facts in a sea of lies.