Moscow and Washington were yesterday engaged in a tense war of words over the conflict in Syria, trading barbed accusations over each other's role in arming opposing sides.

In an indication of the breakdown in international diplomatic efforts to resolve the crisis, the Russian Foreign Minister, Sergey Lavrov, hit back at accusations by the US Secretary of State, Hillary Clinton, that his government was "patently" lying about its weapons shipments to Syria and escalating the situation by supplying attack helicopters to the regime.

Speaking during a visit to Tehran, Mr Lavrov claimed that the US was responsible for an increase in bloodshed, accusing Washington of supplying weapons to the Syrian opposition.

Consensus that the situation on the ground constitutes a full-blown civil war is mounting, with the new French Foreign Minister, Laurent Fabius, yesterday joining the UN peacekeeping chief as defining it as such. Syrian state television yesterday said it had "cleansed" the embattled Latakia town of Al Haffa of "terrorist groups", with troops said to be rounding up scores of men and raiding houses.

Analysts and diplomats say that the Free Syrian Army (FSA) has become better organised and armed in recent months, as weapons shipments begin to seep in from nations such as Saudi Arabia and Qatar. The US State Department has stressed that it is not providing any lethal equipment to rebels, but has reportedly been helping to co-ordinate shipments from the Gulf. Russia is the main arms supplier to Bashar al-Assad's regime, but says its contracts are in line with UN regulations.

"We are not violating any international law in performing these contracts," said Mr Lavrov. "That contrasts with what the United States is doing, which is providing arms to the Syrian opposition which are being used against the Syrian government."

There are fears that the polarisation of the international community – as laid bare in Ms Clinton and Mr Lavrov's comments – could escalate the violence further as foreign powers engage in a proxy battle as diplomatic resolutions remain hamstrung.

"There's a set of international actors, Russia, Iran as well as Hezbollah who have been supplying support to the regime from the beginning and now we've now started to see some support being provided to the rebels," said Salman Shaikh, an expert on Syria at the Doha Brookings Centre.

"The Gulf states are not going to hold back and unless something dramatic happens its going to be a long, bloody and very regretful summer."

There has been a "shocking escalation" in unlawful killings, torture, arbitrary dentition and wanton destruction of homes by government forces and militias, Amnesty International says in a new report today based on visits to 23 towns and villages during April and May.

There are fears for the plight of the population of the Sunni town of al-Haffa, where the FSA said they pulled out overnight yesterday to avoid the killing of more civilians. The city had been under siege for eight days and one wounded FSA fighter who had escaped said troops were making mass arrested and looting houses.