Syria has insisted rebel forces were responsible for an attack in which a team of UN inspectors came under fire while trying to enter the site of an alleged gas attack.
Syrian state television claimed an information ministry source had said the international experts were shot at by "terrorists", a term it commonly uses to describe rebels trying to topple Bashar al-Assad.
"The Syrian government will hold the armed terrorist groups responsible for the safety of the members of the United Nations team," the source added, according to state television.
Syria had agreed to let UN inspectors visit the scene of the violence, which the charity Médecins Sans Frontières said had killed around 350 people and left 3,600 needing treatment for "neurotoxic symptoms".
The team had reportedly arrived in Moadamiyeh, a western suburb of Damascus and one of the areas where the alleged gas attack occurred. They were meeting with doctors and victims at a makeshift hospital when they came under fire.
Martin Nesirky, a spokesman for the UN secretary-general Ban Ki-moon, said the vehicle was "deliberately shot at multiple times" in the buffer zone area between rebel- and government-controlled territory, adding that the team was safe.
Western countries, including Britain, are planning to take unilateral military action against the Assad regime within two weeks in retaliation for its alleged use of chemical weapons on civilians in Syria.
David Cameron discussed launching missile strikes against key regime targets during a 40-minute telephone call with Barack Obama on Saturday night and also with the French President François Hollande on Sunday. While Downing Street said western powers had not ruled out seeking UN endorsement for military action they added that they were also prepared to unilaterally.
"We cannot in the 21st century allow the idea that chemical weapons can be used with impunity and there are no consequences," the foreign secretary, William Hague said. A Downing Street source added: "We intend to show that an attack of this nature will not pass without a serious response."
Mr Cameron is expected to cut short his holiday in Cornwall and return to London to chair a meeting of the Government's National Security Council on Tuesday.
Mr Cameron may hope that the limited nature of the planned response may help him avoid having to hold a pre-emptive vote on military action in Parliament, where he could face strong opposition not just from Labour but also his own backbenches who are concerned about the UK being increasing dragged into yet another Middle Eastern conflict.
A Downing Street spokesman said that Mr Cameron had "always been clear that MPs should have a chance to debate this type of issue" but he added: "He reserves the right for the Government to act and respond."
However Douglas Alexander MP, Labour’s shadow Foreign Secretary said: "If the Prime Minister is now considering military options involving UK personnel then of course I would expect him to seek a recall of Parliament and to come to the House of Commons."
Any military action is likely to take the form of missile strikes from US naval forces in the region, which were ordered to move closer to Syria on Saturday.
The new head of the UK armed forces, General Sir Nick Houghton is meeting with General Lloyd Austin, the US commander in the Middle East in Jordan. The pair are attending a pre-planned meeting of defence chiefs but are expected to discuss the planned action and possible targets "at the fringes" of the conference.
Government sources said it was too early to say whether British military personnel would take part in any of the attacks but would provide complete political support.
The western response it unlikely to be long lasting and is expected to consist of limited air strikes on key targets. It will allow Mr Obama to insist that America is capable and willing to take action over the use of chemical weapons which last year he described as a "red line" for the administration.
However, there are fears that any strike could lead to retaliation or terrorist attacks in the region either from Syria or its Iranian supporters. It is also likely to lead to condemnation from Russia and make any internationally brokered settlement that much harder. Significantly the Government said discussions with Russia over the response to the chemical weapons attack had so far only been at a "senior official" level.
A Downing Street source said that Mr Cameron would speak again to the US President and would also be engaging with other key European allies. It said: "If the Assad regime were innocent they wouldn't have stopped UN inspectors from coming and they would have stopped shelling the area.
"Therefore we are into a scenario of, not has there been an incident and does the international community need to respond, but how should the international community respond? This is where our focus now is.
"The aim here is to have a clear, concrete response from the international community that deters further outrages and makes clear that we will stand up to the prohibition of chemical weapons. We need to show that their use will not go unchecked."
Asked about getting a UN mandate from military action, it said: "We are not excluding the UN route and we will keep engaging with UN partners and working the diplomatic machine. But we do not want the regime or its allies to use the UN to drag this all out. An attack of this nature passes without a serious response."
It added that any attack would not be intended to sway the military balance between Assad forces and they Syrian opposition. "This is not about trying to shape the outcome of the Syrian conflict by military means. This is focused on the incident that happened on Wednesday."
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