International tensions over chemical weapons use in Syria worsened when UN weapons inspectors came under sniper fire as they sought to investigate a deadly attack blamed by Western nations on the Assad regime.
Prime Minister David Cameron is breaking off from his holiday to continue a round of calls with world leaders amid mounting speculation that the UK could join international military action.
He will chair a meeting of the National Security Council on Wednesday to discuss potential responses to the deadly attack which killed hundreds but Foreign Secretary William Hague declined to rule out air strikes within days.
Force may be the only remaining option after the failure of diplomatic efforts to end the bloody violence in Syria, he said, insisting that it could be deployed legally even without Russian support at the UN Security Council.
The inspectors were travelling to the site of the attack in a suburb of Damascus after the regime - which blames any chemical weapon use on opposition forces - gave its permission almost a week after the incident.
Despite a temporary ceasefire being agreed, their first vehicle was "deliberately shot at multiple times by unidentified snipers in the buffer zone area" , a spokesman for UN Secretary-General Ban Ki-moon said.
"As the car was no longer serviceable, the team returned safely back to the government checkpoint. The team will return to the area after replacing the vehicle," he added.
The UN appealed for co-operation from both sides to allow the team access to the site.
The regime already faced criticism for delaying access to the site, making evidence harder to establish.
Syrian President Bashar Assad firmly denies "politically motivated" claims that his regime has used chemical weapons but Mr Hague said there was "no other plausible explanation" for the deaths and injuries.
He said Britain, the US, France and other countries agreed on the need for a "strong response".
Pressed on whether that could mean military action as early as this week, he said: "I am not going to rule anything in or out. I am not going to speculate about that in public."
Mr Hague said diplomatic efforts to resolve the bitter civil war in Syria had failed - and suggested military action could now be the only alternative to allowing chemical weapon use with impunity.
"This may be the choice. This is why we have called for a strong response".
Any intervention would be "in accordance with international law and will be based on legal advice to the National Security Council and to the Cabinet", Mr Hague stressed.
But it could be taken "without complete unity on the UN Security Council", he added, amid frustration over the continued support for the regime from Russia which has blocked previous efforts to secure UN backing.
Action was allowed "based on great humanitarian need", he said.
Mr Cameron is also returning to renewed demands from MPs for Parliament to be recalled before any British involvement in military action.
Tory MP Andrew Bridgen, who has been leading demands for the Commons to be consulted over military action, said MPs should be brought back as early as tomorrow.
Mr Hague said it would "depend on the timing and nature of what we propose to do" and that the Government reserved the right to take swift action if required.
Labour's Thomas Docherty, a member of the Commons Defence Committee, said: "He is leaving open the option of taking military action and then telling Parliament. That is unacceptable."
Russia, which backs the regime and continues to supply it with arms, made clear that it would not back any military intervention, saying that acting without UN Security Council approval would be "a very grave violation of international law".
Foreign Minister Sergei Lavrov said countries talking of military intervention had no evidence that Assad's forces had used chemical weapons and were acting in breach of an agreement at the G8 conference in Northern Ireland in June.
Blaming the controversy on Syrian opposition forces' attempting to derail a planned peace conference, he said Western nations were taking a "dangerous path" which would "sharply aggravate" the situation.
"We have no plans to go to war with anyone," he told a press conference.
"If anybody thinks that bombing and destroying the Syrian military infrastructure, and leaving the battlefield for the opponents of the regime to win, would end everything - that is an illusion."
Former Liberal Democrat leader Sir Menzies Campbell urged caution and backed calls for Parliament to be consulted before the UK joined any action.
"It is inconceivable that any external military action should be taken in Syria before the UN inspectors have completed their investigation and produced a report.
"Equally, if the Prime Minister were to allow UK participation in any such action, he would have to have a very good reason to do so before Parliament has met, discussed and voted on the issue."
Sir Menzies said the situation was different from that of Libya - when Parliament met only after action was taken - because in that case there was United Nations backing for humanitarian intervention.
The ability to act under the "responsibility to protect" doctrine was legally complicated and required a number of criteria to be met, including seeking regional support.
The campaign for pre-approval of military action by Parliament was joined today by a number of senior MPs who tabled a parliamentary motion calling for "a full debate before any British commitment to military action in Syria".
The early day motion - a means of showing the depth of support for issues - was tabled in the name of several chairs of select committees.