The civil war in Syria looks to be entering a new and potentially more dangerous phase, with the alleged use of chemical weapons both escalating the conflict and making foreign intervention more likely. During his visit to Israel, US President Barack Obama said confirmation that chemical weapons had been used in Syria would be a “game changer”.

Dozens of people have been reportedly killed in the city of Aleppo following what is believed to have been a gas attack. When news of the attack first broke, the Syrian government immediately blamed Syrian rebels for the use of chemical weapons. The rebels quickly denied responsibility, saying they had no access to such weapons and had no suitable weapons delivery systems. Obama said he was sceptical of the Syrian government’s claim that the chemical weapons had been used by the rebels.

“Obama is highly unlikely to opt for unilateral on-ground military intervention, given the US is still assessing the cost of being bogged down in Iraq and then Afghanistan.”

UN Secretary-General Ban Ki-moon has called for an investigation into the use of the gas as a weapon in Syria. Obama said: “When you start seeing weapons that can cause potential devastation and mass casualties and you let that genie out of the bottle, then you are looking at potentially even more horrific scenes than we’ve already seen in Syria.”

Obama is now under increasing domestic pressure from both sides of US politics to intervene in the Syrian conflict. He has so far avoided intervention in the hope the war would be resolved through negotiation; however, he said the use of chemical weapons could trigger intervention.

The UK has already started to supply gas masks to Syria’s rebel fighters, along with other humanitarian aid.

If the US intervenes, it is expected to be under the rubric of the “Responsibility to Protect” policy (R2P). Under R2P, states have a responsibility to intervene in domestic conflicts when a government cannot protect its people from, or engages in, war crimes, crimes against humanity or other mass human rights abuses. There are several levels of invoking R2P, only the last of which is military intervention. Ordinarily, R2P requires the approval of the UN Security Council. However, Russia and China have both made clear they will not allow the Security Council to invoke R2P on this issue.

Obama is highly unlikely to opt for unilateral on-ground military intervention, given the US is still assessing the cost of being bogged down in Iraq and then Afghanistan. However, there is some possibility he could authorise selective air strikes against chemical weapons sights and related targets, or reach agreement with other NATO states to allow such strikes.

This would probably be less than NATO’s barrage of air strikes in Kosovo in 1999, which ended Serbian occupation, and probably less than the air support for anti-Muammar Gaddafi forces in Libya in 2011. In particular, the US would be particularly concerned not to provoke Russia into also intervening, thus turning the Syrian civil war into a war by proxy between major powers.

The US is also concerned to eliminate chemical weapons from a post-Bashar al-Assad strategic equation. It fears if the Assad regime falls, weapons could fall into the hands of the radical Islamist groups that are carrying much of the anti-Assad effort. This, too, may prompt the use of air strikes.

Along with other Western states, the US wants to see an end to the Syrian conflict. But few want to see the rise of a militant Islamist state, especially one that demonstrably has access to “weapons of mass destruction”.

* Professor Damien Kingsbury is director of the Centre for Citizenship, Development and Human Rights at Deakin University.