The US secretary of state accuses Moscow of escalating the conflict in Syria by supplying the Assad regime with attack helicopters Reuters

The crisis in Syria has deepened as the US accused Russia of sending attack helicopters that could be used by President Bashar al-Assad to help crush the rebellion against his regime.

On the day that a senior UN official declared Syria was in a state of civil war, Hillary Clinton, the US secretary of state, warned in Washington that the Russian aircraft would "escalate the conflict quite dramatically". The US and other western countries have been trying desperately to get Moscow to put pressure on Assad amid a recent surge in violence.

Fears of a further sharp deterioration were fuelled by the statement from Herve Ladsous, head of the UN's department of peacekeeping operations, who said that Syria was, in effect, already in a state of civil war after 15 months of violence and an estimated 15,000 dead.

"Clearly what is happening is the government of Syria lost some large chunks of territory, several cities to the opposition, and wants to retake control," he said.

It was thought to be the first time a senior UN official has characterised the conflict as a civil war. "Now we have confirmed reports of not only of the use of tanks and artillery but also attack helicopters," Ladsous told reporters in New York. "This is really becoming large scale."

Attacks on UN monitors added to now routine reports of bloodshed across the country and heightened a sense of international urgency that has been growing since two massacres blamed on the regime in recent weeks. It has said "armed terrorists" were responsible.

The Syrian Revolution General Commission reported 36 people killed on Tuesday, mostly as a result of heavy shelling of the central city of Homs. Residents of the city's Khalidiya neighbourhood told al-Jazeera TV they had been trapped in their homes for the last three days without electricity or water. Videos from the city showed explosions in built-up areas.

The International Committee of the Red Cross said the humanitarian situation was deteriorating in several parts of the country, making it impossible to respond to everyone's needs at once.

Hopes are fading that the six-point peace plan promoted by Kofi Annan – the only such scheme available – can be implemented in the absence of a ceasefire and political will by the regime and its enemies. Annan has asked governments with influence to "twist arms" to halt the escalating violence, his spokesman said.

Clinton's statement about Russian helicopters was clearly no throwaway remark and was almost certainly based on intelligence, diplomats said. "We have confronted the Russians about stopping their continued arms shipments to Syria," she said. "They have from time to time said that we shouldn't worry, everything they're shipping is unrelated to [Syrian] actions internally. That's patently untrue and we are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria which will escalate the conflict quite dramatically." Russia, a long-term ally of Syria, has resisted western appeals to apply pressure on Assad but insists it is not supplying weapons that would help crush the uprising. However, attack helicopters clearly would.

The UN and Syrian opposition sources have described the more frequent use of helicopters in recent days in attacks across the country. This could, in theory, be used to drum up support for the imposition of a no-fly zone over Syria – similar to the one created over Libya before the Nato intervention last year. Russia would be highly likely to resist any such idea. Clinton also referred to a massing of Syrian forces around the country's second city, Aleppo. "That could very well be a red line for the Turks in terms of their strategic or national interests," she warned.

British officials monitoring the crisis expressed alarm about the latest developments, reflected in a hard-hitting speech by William Hague, the foreign secretary, on Monday in which he referred to "grotesque crimes" committed by the Syrian government. Elements of concern include the massacres at Houla and al-Qubair – 108 and 78 people reported killed respectively; heavy bombardments of cities followed by attacks by shabiha militamen; more effective fighting tactics by the rebels of the Free Syrian Army and the "lack of traction" for the Annan plan. "It is very clear that the cycle of violence is intensifying and that is deeply concerning," said an FCO spokesman. Syria, meanwhile, accused the US of "blatant interference" in its affairs and supporting terrorist crimes. The complaint was linked to warnings from Washington that a new massacre was imminent in al-Haffa, a rebel-held village near Assad's hometown of Qardaha.

"Those who support armed groups and provide them with funds and weapons and cover up their crimes are directly complicit in the shedding of Syrian blood, no matter what statements they make," said the foreign ministry in Damascus.

The UN said members of its 300-strong observer team in Syria were attacked by angry crowds as they tried to reach al-Haffa. The vehicles also were later shot at though no observers were hurt.