Iran military chief warns U.S. of 'severe consequences' if it intervenes in Syrian civil war as Cameron and Obama discuss crisis Deputy chief of staff of Iran's armed forces Massoud Jazayeri issued the warning to the U.S. Comes after Barack Obama pledged a 'serious response' if Bashar Assad regime is proved to have used chemical weapons Aid organisations claimed this week that more than 350 people - many of them children - died due to the effects of neurotoxins Iran has warned the United States it will face 'severe consequences' if it crosses the 'red line' on Syria. The threat came after Barack Obama pledged a 'serious response' if Bashar Assad regime is proved to have used chemical weapons. Deputy chief of staff of Iran's armed forces Massoud Jazayeri said: 'America knows the limitation of the red line of the Syrian front and any crossing of Syria's red line will have severe consequences for the White House.' Warning: Iran has warned the U.S. President Barack Obama, pictured on Friday as he walked across the South Lawn of the White House, of 'severe consequences' should America cross the 'red line' on Syria Prepared: Chuck Hagel, right, says the President has ordered the Pentagon to prepare potential military options Planning: Prime Minister David Cameron and U.S. President Barack Obama, pictured in May this year, have both ordered officials to examine 'all options' on Syria President Obama has been holding talks with David Cameron over the growing crisis in Syria. The Prime Minister and the U.S. President discussed the conflict by telephone last night and have ordered officials to examine 'all options'. The talks came amid claims from aid organisations that more than 350 people - many of them children - died due to the effects of neurotoxins this week. A Downing Street spokesman said the leaders spoke for around half an hour. 'They are both gravely concerned by the attack that took place in Damascus on Wednesday and the increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people,' he said. The UN Security Council has called for immediate access for UN investigators on the ground in Damascus. The fact that President Assad has failed to co-operate with the UN suggests that the regime has something to hide. Chemical attack: Aid organisations claim the more than 350 people - many of them children - died after being exposed to neurotoxins in Arbeen town, Damascus, Syria, this week They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options. They agreed that it is vital that the world upholds the prohibition on the use of chemical weapons and deters further outrages. They agreed to keep in close contact on the issue. President Obama previously suggested that the use of chemical weapons in Syria would be a 'gamechanger'. But he has stopped short of committing to direct intervention in the troubled country - a step that would risk inflaming tensions with Russia. Mr Cameron would also face domestic resistance to military action, with Tory MPs already having insisted there should be a Commons vote before arms are supplied to rebels. However, there is speculation that the U.S., Britain and France could back limited airstrikes to demonstrate that deployment of chemical weapons will not be tolerated. Mr Cameron also spoke to Canadian PM Stephen Harper, who agreed that the 'international community must respond appropriately'. Medecins Sans Frontieres said hospitals it supports in Syria treated some 3,600 patients with 'neurotoxic symptoms', 355 of whom died. It said the patients had arrived in three hospitals in the Damascus area on Wednesday. Staff described people suffering from convulsions, extreme salivation, contracted pupils and sight and respiratory problems. However, the organisation stressed it could not 'scientifically confirm' the use of chemical weapons. Escalation: Angela Kane, the UN's high representative for disarmament, arrives in Damascus, Syria on Saturday to press President Bashar Assad to let UN investigators look into the reported chemical attack