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By Alastair Jamieson, Courtney Kube, Erin McClam and Alexander Smith, NBC News

The British Parliament Thursday rejected a proposal for military action in Syria -- while the Obama administration said it would make its own decision on a possible strike.

UK Prime Minister David Cameron indicated he won’t proceed without parliamentary approval, saying the government "will act accordingly."

"I strongly believe in the need for a tough response in the use of chemical weapons but I also believe in respecting the will of this House of Commons," he said in the aftermath of his defeat.

Thursday vote was nonbinding, but Cameron's loss on even a symbolic vote likely means there will be no second-round vote next week. 

Facing not only the pushback from Britain but also a growing hesitation in Congress, the Obama administration insisted Thursday that any attack on Syria would be limited -- and flatly rejected comparisons to the Iraq war, where supposed weapons of mass destruction were never found.

"President (Barack) Obama's decision-making will be guided by what is in the best interests of the United States," National Security Council spokeswoman Caitlin Hayden said following the UK vote. "He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable." 

At a news conference in Manila, Philippines, Defense Secretary Chuck Hagel said, "The British have been very strong in condemning the Syrian regime's use of chemical weapons, and that vote in the Parliament doesn't change that. That is a very significant position for any nation to take publicly. We'll continue to work with Britain and  consult with Britain as we are with all our allies."

Hagel added: "It is the goal of President Obama and our government to whatever decision is taken that it be an international collaboration and effort. ... Our approach is to continue to find an international coalition that will act together."

Amid mounting skepticism from lawmakers who question the prudence of U.S. involvement in Syria's protracted civil war, administration brass offered House and Senate leaders and committee chairmen and ranking members their latest assessment of the situation in Syria Thursday night. 

A State Department spokeswoman, Marie Harf, said Iraq and Syria were “in no way analogous.”

Obama has said he has not decided on a military strike against Syria, which would probably come with cruise missile strikes form the Mediterranean Sea. But Harf made clear that the administration would decide its course.

“We make our own decisions and our own timeline,” she said.

White House spokesman Josh Earnest said at a briefing Thursday, "When the president reaches a determination about the appropriate response ... and a legal justification is required to substantiate or to back up that decision, we'll produce one on our own."

Pentagon officials insist strikes are likely “within days” and that the U.S. was “past the point of no return” on the issue -- suggesting Washington was prepared to act unilaterally.

The timing of any strike is also complicated by the presence in Syria of United Nations inspectors collecting samples from the scene of the suspected Aug. 21 attack on rebel-held suburbs east of Damascus.

U.N. Secretary General Ban Ki-moon said Thursday that his team would not leave until Saturday morning, and confirmed he had spoken to Obama about impending action.

Concern in Britain stems from memories of the country’s support for the 2003 U.S.-led overthrow of Saddam Hussein, which was based on evidence of weapons of mass destruction that later turned out to be false, prompting public anger. 

However, Cameron assured lawmakers Thursday: "We must not let the specter of previous mistakes paralyze our ability to stand up for what is right.

“We must not be so afraid of doing anything, that we end up doing nothing. Let me repeat again, there will be no action without a further vote in this House of Commons, but on this issue Britain should not stand aside."

Russia announced it was sending an anti-submarine ship and a missile cruiser to the Mediterranean, according to an Interfax news agency report cited by Reuters.

As the crisis deepened, the International Committee of the Red Cross (ICRC) warned that any escalation of Syria's civil war would deepen shortages of medicine, food and water in rural areas.

"The suffering of civilians in Syria has now reached unprecedented levels, and there appears to be no end in sight," said Magne Barth, head of the ICRC’s delegation in the country, in a statement. "Further escalation will likely trigger more displacement and add to humanitarian needs, which are already immense.”

British legislators were sent two separate open letters Thursday, one from Syrian opposition groups, the other from the speaker in the Syrian parliament, both appealing for action.

"We call upon you to send a message to the regime that it cannot gas sleeping children with impunity," the National Coalition of Syrian Revolution and Opposition Forces said in its letter.

Jihad Allaham, the speaker in the Syrian People's Assembly, invited U.K. MPs to Syria. "We implore you to communicate through civilized dialogue rather than a monologue of blood and fire," he said in his letter to John Bercow, the speaker of the House of Commons.

In a parallel debate in the upper chamber of Britain's parliament, the House of Lords, veteran lawmaker Lord Paddy Ashdown said: “I remember well trying to persuade parliament that we needed to intervene in Bosnia - when we were under shadow of Vietnam. Now here we are and I think living under the shadow of Iraq.

"But this is not Iraq, we are not putting boots on the ground and we are not invading and above all this is not George W. Bush, it's Obama."

NBC News' Andrea Mitchell, Jim Miklaszewski, Mohammed Abbas, Ayman Mohyeldin and Becky Bratu contributed to this report.