WASHINGTON – A visibly angry Secretary of State John Kerry said on Monday that it was “undeniable” the Assad regime has used chemical weapons on a massive scale, and that US President Barack Obama would respond strongly.“This is about the large-scale, indiscriminate use of weapons that the civilized world, long ago, decided must never be used at all,” Kerry said.Reuters contributed to this report.He said that, as a father, “the images of entire families dead in their beds without a drop of blood, or even a visible wound” shook his consciousness and that of the nation.He said an attack of this “staggering scale could not be contrived or fabricated.”“Make no mistake,” Kerry warned, “President Obama believes there must be accountability for those who would use the world’s most heinous weapons against the world’s most vulnerable people. Nothing today is more serious, and nothing is receiving more serious scrutiny.”The United States is preparing legal justification for the use of force against Syria with allies Britain and France that would circumvent the United Nations Security Council, where Russia and China have vowed to block any resolution authorizing military intervention in the conflict.The US will detail its case soon, with military action possible in the coming days, sources tell The Jerusalem Post.On Monday, the US dismissed Syria’s decision to grant UN inspectors access to the Damascus suburb of Ghouta, where Syrian rebels claim Assad’s army used sarin gas last week to kill hundreds of civilians as they slept.The UN chemical weapons experts interviewed and took blood samples on Monday from victims of last week’s apparent poison gas attack in a rebel-held suburb of Syria’s capital, after the inspectors themselves survived sniper fire that hit their convoy.The United Nations said one vehicle in its convoy was crippled by gunshots fired by “unidentified snipers.” The team continued on after turning back for a replacement car.The inspectors had been stuck in a downtown luxury hotel since the attack, waiting five days for government permission to visit the scene a few miles away. They had arrived three days before the incident, with a mandate to investigate earlier, smaller reports of chemical weapons use.The investigation that started Monday would be “too late to be credible,” the US said, as evidence of sarin lasts for a maximum of 10 days.Russia has repeatedly warned the West against action in recent days, citing international law as prohibitive to any trilateral campaign.Last week, Obama stated that action in Syria “without a UN mandate” might indeed violate international law.But the Obama administration has proven creative in its legal justifications in the past.Faced with a domestic law requiring the suspension of aid to any country that has faced a military coup d’etat, the president made the determination not to make a determination on whether a coup had occurred in Egypt in July, after the military forcibly removed its president, Mohamed Morsi, from power.Administration officials have been studying Western intervention in Kosovo as a model for action in Syria, The New York Times reported Monday.Obama, British Prime Minister David Cameron and French President Francois Hollande all spoke to each other and to other allies in the past few days in a flurry of phone calls. Cameron also called Russian President Vladimir Putin on Monday.Several NATO countries have issued statements pledging a response, although none has been specific about what is planned.Top military officers of the United States, Britain, France, other NATO allies and the main anti-Assad countries in the region, including Saudi Arabia and Turkey, met in Jordan on Monday to discuss Syria, diplomats there said.The conference was pre-planned but had taken on new significance because of the latest events, the diplomats said.Speaking in Indonesia, US Defense Secretary Chuck Hagel refused to detail military contingencies prepared by the Pentagon for the president.“We are analyzing the intelligence,” Hagel said. “And we will get the facts. And if there is any action taken, it will be in concert with the international community and within the framework of legal justification.”Ten Downing Street told Britain’s Parliament on Monday that Cameron reserved the right to take action without parliamentary approval, as lawmakers there considered returning to London early from summer recess.Britain’s foreign minister, William Hague, told local news outlets that the UK did not see Security Council approval as a requirement for action.“Otherwise it might be impossible to respond to such outrages, such crimes, and I don’t think that’s an acceptable situation,” Hague said when asked about unanimous Security Council approval by BBC Radio.Britain has readied Royal Navy forces in the Mediterranean to strike with American destroyers, should their leaders choose to move forward. A strike would likely involve standoff assets that would avoid entry into foreign air space.Senator John McCain, anticipating a limited, symbolic strike on Assad’s command and control centers, said Sunday that the president should consider “very serious action, not just launching some cruise missiles.”France’s Francis Hollande on Sunday said that his country’s intelligence agency had gathered corroborating evidence that the Assad regime had used chemical weapons against civilians on August 21.US officials have said there is “little doubt” that an attack occurred and was perpetrated by Assad’s forces.The State Department has repeatedly asserted that the US does not believe rebel forces in Syria are capable of either making or dispersing chemical agents on a mass scale, much less sarin, which is difficult to handle.Sarin gas can evaporate quickly after release, especially under hot conditions such as August in Syria.Western officials were also concerned when Assad forces continued shelling the scene with conventional artillery after Wednesday’s alleged chemical attack, perhaps in an effort to destroy evidence.Syrian rebel leaders told Reuters on Monday that peace talks were no longer on the table after Wednesday’s attack. Syrian National Coalition secretary-general Badr Jamous said they “refused to speak about [peace talks in] Geneva after what’s happened,” and called Assad “Bashar the Chemist.”“We cannot allow impunity in what appears to be a grave crime against humanity,” UN secretary-general Ban Ki-moon said of the crisis on Sunday.International law bans the use of chemical weapons on any battlefield under any circumstances.And the “responsibility to protect,” or R2P – a norm agreed upon by global powers at the United Nations 2005 World Summit– compels the international community to respond if a country fails to protect its citizens from genocide, war crimes, ethnic cleansing or crimes against humanity.R2P is not law. But its principles are rooted in law and have been agreed upon by the international community, including by Russia, which has cited R2P in the past when waging military campaigns in the Caucases.