As Syria descends deeper into civil war and human misery, pressure for yet another western military intervention in the Arab world is growing. Last week, General Martin Dempsey, chairman of the US joint chiefs of staff, declared that the US might take the "military option" in Syria if it was "asked to do so". Barack Obama's Republican rival Mitt Romney is meanwhile demanding that the US government arm the Syrian opposition.

Today, Russian and Chinese leaders reaffirmed their opposition to forced regime change and support for UN envoy Kofi Annan's peace plan. But Susan Rice, US ambassador to the UN, has made clear western powers might act alone and take action "outside the authority" of the UN. Even the new French president François Hollande has said military intervention in his country's former colonial territory was "not to be ruled out".

The latest calls for action against Bashar al-Assad's regime follow the slaughter of 108 people, including 49 children, in Houla less than a fortnight ago. Opposition activists have blamed pro-regime "shabiha" sectarian militias for the massacre; the government al-Qaida terrorists. But there's no doubt that atrocities such as Houla – let alone killings on a larger scale – have the potential to turn intervention grandstanding into the real thing.

That's what happened in Kosovo 13 years ago, when contested killings in Racak led to Nato's bombing campaign outside the authority of the UN. The US administration continues to resist demands for open intervention in Syria. But Hillary Clinton says the case for intervention is getting stronger "every day", while the opposition Free Syria Army has now declared itself "free of any commitment" to the UN peace plan.

The reality is that intervention in Syria by the US and its allies has already begun. The western powers have backed the fractious opposition Syrian National Council since the early days of last year's uprising. So have the Gulf autocracies led by Saudi Arabia, who have stepped up the flow of weapons and cash to favoured Syrian rebel groups in recent months, while Turkey has provided a cross-border base. That is co-ordinated with the US, which supplies the same groups with "non-lethal assistance" and "communications equipment".

In other words, the US and its allies are sponsoring regime change through civil war. And while paying lip service to the Annan plan for demilitarisation and negotiation, they are making sure it won't succeed. The results can be seen on the ground. Overall, lethal violence is estimated by human rights groups to have dropped by 36% since the plan was supposed to come into effect, but government casualties have increased sharply over the same period (953 reported killed since mid-March). Rebel fighters claimed to have killed 80 government troops last weekend alone.

Syria is reported by the western and Gulf-controlled Arab media through the prism of a popular uprising against an authoritarian regime. But that is only one vital dimension of the conflict. And as brutal repression by a government which retains significant support has been met with a growing armed campaign, grassroots opposition has been displaced by foreign-backed groups whose strategy to win power is based on engineering outside intervention.

It has also increasingly morphed into a sectarian conflict, as the Alawite-dominated regime has used minorities' fears of a Sunni-dominated opposition to bolster support. The latest phase of Sunni-Shia conflict in the Middle East owes its virulence to the occupation of Iraq, where the US ruthlessly played the sectarian card to prevent the emergence of a genuinely national resistance. It has also been a knife at the heart of the Arab revolution and the linchpin of the Saudi-led strategy to prevent uprisings engulfing the conservative Gulf regimes.

Anti-Shia incitement has been central to Saudi propaganda against reform in the kingdom itself, the crushing of democratic protest in Bahrain and the drive to focus opposition across the region against Damascus (Alawites being a quasi-Shia sect), rather than Amman or Riyadh. It's also what has attracted al-Qaida and other Sunni volunteers to join the fight against the Assad regime, as tit-for-tat confessional killings multiply. For Syria and neighbouring Lebanon, with their precarious ethnic and religious makeups, that is a disaster.

But it is the third dimension of the crisis – Syria's role as Iran's principal ally – that gives it the potential to set the region on fire and draw the outside world into a devastating conflict. The internal struggle in Syria, whose territory has been occupied by Israel for the last 45 years, has already become part of a western and Saudi proxy war against Iran and its Lebanese ally Hezbollah. As James Rubin, US assistant secretary of state under Bill Clinton, claimed this week, US intervention in Syria would be a "risk worth taking" because Iran "would no longer have a Mediterranean foothold from which to threaten Israel and destabilise the region".

In fact, Iran's alliance with Syria is one more reason why increasing western and Gulf dictators' intervention in Syria would escalate the conflict, not end it. Last year's Nato intervention in Libya increased the death toll by a factor of 10 to 15 and left a country of lawless warlords, torture and ethnic cleansing. Intervention in Syria, whether by fully arming the opposition or using air power to create "humanitarian corridors", would have a far more devastating impact.

That's partly because the Syrian regime has significant air defences and large-scale armed forces and the conflict is being fought out in heavily populated areas. But it's also because of the sectarian schisms and the risk of spreading the conflict further into countries such as Iraq and Lebanon. Why the states that brought blood and destruction to Iraq and Afghanistan should be thought suitable vehicles of humanitarian deliverance to Syria is a mystery. But full-scale foreign intervention would certainly lead to a far greater civilian death toll and many more Houlas.

Right now, lower-level intervention is bleeding Syria in a war of attrition. Short of an internal coup, the only way out of a deepening sectarian and regional conflict is an internationally guaranteed negotiated settlement that allows Syrians the chance to determine their own future. That means the US and its allies giving the Annan plan a chance, as much as Iranian and Russian pressure on Damascus. The consequences of the alternative – full-scale military intervention – would be incalculable.

Twitter: @SeumasMilne