(CNN) -- British Prime Minister David Cameron opened an emergency session of the House of Commons Thursday by saying the debate on Syria is about "how to respond to one of most abhorrent uses of chemical weapons in a century" -- not about regime change or invasion.

"Put simply, is it in Britain's national interest in maintaining an international taboo against the use of chemical weapons on the battlefield?" Cameron asked lawmakers. "I would say yes it is."

Cameron told members of the House of Commons -- whom he recalled from summer vacation to debate a British response to the deaths of hundreds in a chemical weapons attack outside Damascus last week -- that the government would not act without first hearing from U.N. weapons inspectors, giving the United Nations a chance to weigh in and Parliament to have another vote.

But, he said, failing ultimately to act would give Syrian President Bashar al-Assad the unmistakable signal that he could use such weapons "with impunity," Cameron said.

Cameron's government published an intelligence assessment Thursday that concluded it was "highly likely" that the Syrian government was behind last week's suspected poison gas attack. British intelligence said at least 350 people died, while rebel leaders have put the death toll at more than 1,300.

"It is not possible for the opposition to have carried out a (chemical weapons) attack on this scale," the British Joint Intelligence Organisation said in the assessment.

As the British debated Cameron's call, the United States moved another destroyer into the eastern Mediterranean Sea in preparation for possible airstrikes, joining four others already off Syria, a Pentagon official told CNN.

The British dossier on Syria also concluded the Syrian government had used chemical weapons on 14 previous occasions. Cameron said he believes al-Assad opted to increase the scale of his chemical attacks as a sort of test for the world.

"He wants to know whether the world will respond to the use of these weapons," the prime minister said.

But memories of more than a decade of bruising warfare in Iraq and Afghanistan hung over the debate, with many members sounding uneasy about committing British forces to another Middle Eastern conflict. The government said it could justify the use of force against Syria on humanitarian grounds, to stop the suffering, even if the United Nations declined to authorize a strike.

"The use of chemical weapons by the Syrian regime is a serious crime of international concern, as a breach of the customary international law prohibition on use of chemical weapons, and amounts to a war crime and a crime against humanity. However, the legal basis for military action would be humanitarian intervention; the aim is to relieve humanitarian suffering by deterring or disrupting the further use of chemical weapons," the government said in a statement released Thursday.

Syria's government offered its own arguments against such an intervention. In an open letter to British lawmakers expected to vote Thursday on a motion blocking military action without a U.N. resolution, the speaker of Syria's parliament riffed on British literary hero William Shakespeare, saying: "If you bomb us, shall we not bleed?"

He also offered a veiled warning to the UK, comparing the current situation to the march to war against Iraq a decade ago.

"Those who want to send others to fight will talk in the Commons of the casualties in the Syrian conflict. But before you rush over the cliffs of war, would it not be wise to pause? Remember the thousands of British soldiers killed and maimed in Afghanistan and Iraq, not to mention the hundreds of thousands of Iraqi dead, both in the war and in the continuing chaos."

British Commons Speaker John Bercow published the letter, which was dated Thursday.

The releases come as British lawmakers appeared to put the brakes on any potential plans to strike out at Syria over its use of chemical weapons.

"It certainly seemed 48 hours ago that there was an all-party consensus that parliament today would be endorsing the bombing of Syria this weekend, and I think people have pulled back from that," Parliamentarian Diane Abbott of the Labour Party said.

"It's not clear (whether) a bombing mission like that would be legal ... and it's not clear that it would make things better."

Britain's Parliament will vote on a motion Thursday that would rule out the idea of possible military action until the U.N. inspectors reveal their findings to the U.N. Security Council.

After United Nations inspectors currently in Syria looking for evidence of chemical weapons have made their findings, members of Parliament would be required to take another vote, according to the motion being put forward.

The inspectors are expected to leave Syria by Saturday morning, U.N. Secretary-General Ban Ki-moon said Thursday, according to his spokesman.

'We have looked at all the evidence'

Other Western countries are also mulling possible military action against Syrian forces after the alleged chemical assault near Damascus on August 21. Death toll estimates from that day range from several hundred to 1,300.

On Wednesday, U.S. President Barack Obama said there's no doubt that Syria launched chemical weapons attacks against its own people.

Syrian President Bashar al-Assad's regime has blamed rebels for the attack, a claim that Obama said was impossible.

"We have looked at all the evidence, and we do not believe the opposition possessed ... chemical weapons of that sort," he told "PBS NewsHour" Wednesday.

"We do not believe that, given the delivery systems, using rockets, that the opposition could have carried out these attacks. We have concluded that the Syrian government in fact carried these out. And if that's so, then there need to be international consequences."

The British assessment published Thursday agreed with those conclusions.

Obama said that he has not made a decision about whether to conduct a military strike in Syria.

A senior administration official said the United States would continue to consult with British officials, but declined to say if the slowdown in London would affect U.S. decision making on Syria.

Meanwhile, al-Assad vowed to defend his country against any outside attack.

"The threats of launching an aggression against Syria will increase its commitments to its rooted principles and its independent decision that originated from will of its people, and Syria will defend itself against any aggression," the president said Thursday in a speech to Yemeni politicians.

Bashar Jaafari, Syria's ambassador to the United Nations, said rebels were to blame for the chemical attacks. He accused opposition fighters of getting materials to produce chemical weapons "from outside powers -- mainly speaking, Turkey, Saudi Arabia and Qatar."

Rows of corpses

Last week wasn't the first time reports surfaced of a chemical weapons attack in Syria. But it was by far the worst.

"Syria is now undoubtedly the most serious crisis facing the international community," Lakhdar Brahimi, U.N. and Arab League special envoy to Syria, said Wednesday in Geneva.

"It does seem that some kind of substance was used that killed a lot of people," Brahimi said.

The death toll could be in the hundreds, or possibly more than a thousand, he explained. Opposition activists say 1,300 people were killed.

Images show rows of corpses -- including those of children -- lined up in a room. The bodies had no outward signs of trauma.

Those who said they survived the alleged chemical attack described a horrific scene in the town of Zamalka.

"After the chemicals, they woke us up and told us to put masks on," a 6-year-old boy said.

"I told my dad I can't breathe. My father then fainted and I fainted right after that, but we were found and taken to the emergency room."

CNN obtained video of the boy and others who made the claims to a journalist in the area.

No matter what U.N. investigators say really happened in Zamalka, it's only one scene in the civil war that has killed more than 100,000 people since 2011. Many of those killed were civilians.

CNN's Max Foster and Bharati Naik contributed to this report.