Military action against Syria may be the only remaining response to the suspected large-scale chemical weapons attack by the Assad regime, William Hague has said.

The Foreign Secretary declined to be drawn on the options being considered by western allies but would not rule out the possibility of air strikes or other measures being taken within days.

And he told MPs demanding the recall of Parliament from its summer break ahead of any British involvement that it would "depend on the timing and nature of what we propose to do".

Prime Minister David Cameron is cutting short a family holiday in Cornwall to return to Downing Street as he continues a sustained round of telephone diplomacy with fellow leaders to secure agreement on a response. He will chair a meeting of the National Security Council on Wednesday to discuss the next steps. But he also returns to mounting pressure from both Tory backbenchers and the Labour Opposition for MPs to be summoned back to Westminster to pre-approve any military action involving UK personnel.

United Nations inspectors visiting the area of Damascus where chemical weapons were apparently deployed have come under attack. A statement from the UN on Twitter said: "Vehicle of @UN #Syria #ChemicalWeapons team hit by sniper fire. Team replacing vehicle & then returning to area." The team was given access to the area after the regime gave permission and a temporary ceasefire was agreed.

Syrian president Bashar Assad says the claims are "politically motivated" and defy logic as the regime has forces near the area - and warned in a Russian newspaper that any US-led military action would end in failure.

Mr Hague insisted there was "no other plausible explanation" and accused Damascus of delaying the arrival of the UN team to reduce the chances of them finding evidence. 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".

The UN made no mention of any casualties, but said the inspection team was shot at "multiple times". A statement said: "The first vehicle of the chemical weapons investigation team was deliberately shot at multiple times by unidentified snipers in the buffer zone area. As the car was no longer serviceable, the team returned safely back to the government check-point. The team will return to the area after replacing the vehicle. It has to be stressed again that all sides need to extend their co-operation so that the team can safely carry out their important work."

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".