The US navy is deploying an extra missile warship to the eastern Mediterranean ahead of a summit to debate last week's chemical weapons attack in Syria, as calls mount for a military response.

The summit, to be held in Jordan's capital Amman in the first half of the week, has been called as a consensus emerges that the nerve agent sarin was the cause of hundreds of deaths in rebel-held east Damascus early on Thursday.

Biological samples taken from victims and survivors of the attack have now been passed to western officials in Jordan after having been smuggled out of Syria over the past 72 hours. Unmarked questionnaires have been distributed to officials in the three most affected communities, asking for forensic and environmental details, as well as for organ tissue and clothing worn by victims. A final death toll has not been established, with estimates ranging from several hundred to more than 1,400.

Three hospitals in Syria's Damascus governorate which are supported by the international medical humanitarian organisation Médecins sans Frontières reported that they received approximately 3,600 patients displaying neurotoxic symptoms in less than three hours on the morning of Wednesday, 21 August. Of those patients, 355 are reported to have died.

Officials, who have not identified themselves but claim to be part of an international response, have also made phone contact with rebel officials, seeking photographs of the rockets that are thought to have carried the gas. The remains of 20 such rockets have been found in the affected areas, activists and local residents say. Many remain mostly intact, suggesting that they did not detonate on impact and potentially dispersed gas before hitting the ground.

Survivors spoken to by the Observer last week say they heard the rockets whistling in but they did not detonate. Several have been photographed half buried in bitumen, which suggests that they did not carry a warhead or, if they did, it failed to explode.

France, Britain and Turkey have blamed the Syrian regime for the attack, which came as its military forces were advancing into the area. France's foreign minister, Laurent Fabius, said on Saturday that "all the information at our disposal converges to indicate that there was a chemical massacre near Damascus and that the [regime of Bashar al-Assad] is responsible". The foreign secretary, William Hague, said last week that "this is a chemical attack by the Assad regime" and it was "not something that a humane or civilised world can ignore". Barack Obama has described the incident as "grave" and asked intelligence officials to prepare a detailed assessment.

Syria has continued to deny responsibility as the UN's disarmament chief, Angela Kane, arrived in Damascus to try to negotiate access to the site of the attack for an inspection team that was sent to investigate three earlier alleged attacks. The team has been in the capital for the past six days and has been pressing for permission to make the journey – only a short distance from their hotel – for the past 72 hours. Rebel groups in the area say that they will guarantee safe passage. However, the Syrian government has not agreed and the UN fears that the journey is unsafe without a negotiated agreement. Syrian state television said on Saturday that its forces had found tunnels in rebel areas in which chemicals were stored.

The images of dead and dying in the attack have caught the attention of regional governments like nothing else in the two-and-a-half year conflict. The chairman of the US joint chiefs of staff, General Martin Dempsey, will travel to Jordan along with the head of the US central command, General Lloyd Austin, and chiefs of staff from Turkey, Britain, France, Qatar, Canada, Saudi Arabia, Italy and Canada.

The addition of a US destroyer takes to four the US Mediterranean flotilla, one more than normal. US defence secretary Chuck Hagel said no decision has been made to use the warships in operations against Syria. Speaking on Friday, officials in Washington said no response to Syria would involve sending troops into the country. Two of Syria's three main allies, Russia and Iran, have supported calls for a transparent and credible inquiry into the attack. Both accuse rebel groups of having carried out the atrocity. The Lebanese militia, Hezbollah, has remained silent since Thursday.

Hezbollah leaders roundly condemned car-bombings of two mosques in Lebanon's second city, Tripoli, one day later, which killed 42 and wounded hundreds. The mosques had been focal points of anti-Assad rhetoric in the largely Sunni north. Lebanon, an unstable multi-confessional state, has been perenially on edge since the start of the Syrian uprising with occasional flare-ups in violence that threaten to drag it into the chaos consuming its powerful neighbour.