In a rare warning against Syrian President Bashar al-Assad’s regime, it said Lebanese troops “took the necessary defensive measures to respond immediately to any similar violations”.

Hague told a joint news conference that Britain and its allies must be “prepared to do more” to save lives in Syria, calling the conflict “the most urgent crisis anywhere in the world today” and a grave regional threat.

Kerry insisted the goal of diplomatic efforts was to reach a political solution and transition of power that “gives the Syrian people the chance to have a new beginning where they choose their future leadership”.

Later Wednesday the US State Department announced the easing of restrictions on trade with Syria’s opposition, in a move aimed at helping supply the critical needs of Syrians in “liberated areas”.

The new waivers allow companies to supply software, technology, reconstruction and power generation equipment, as well as farm and food production equipment, to opposition-controlled areas.

Private sales of arms to the rebels, however, were not included in the new policy.

Lebanon’s President Michel Sleiman called the Arsal attack “a violation of Lebanon’s sovereignty” and said Beirut has the right to take steps to defend itself.

Hours after the helicopter gunship strike Syria’s army command insisted that it respected its neighbour’s sovereignty but would continue to target rebels, state news agency SANA said.

Most residents of Arsal, in the hills just 12 kilometres (7.5 miles) from the border with Syria, support the Sunni-led uprising against the Assad regime.

The majority of Syria’s population is Sunni but it has been ruled for more than 40 years by the Assad clan from the Alawite community, an offshoot of Shiite Islam.

The Syria conflict erupted in March 2011 following a bloody regime crackdown on Arab Spring-inspired democracy protests.

Since then, there have been several spillover attacks in Lebanon involving both sides in Syria, but Wednesday’s army statement was the first of its kind in the nearly 27-month conflict.

Lebanon’s poorly equipped army normally coordinates closely with the Syrian military.

Damascus dominated Lebanon politically and militarily for 30 years until 2005, and still exerts significant influence through its allies in the country.

Arsal has been used as a conduit for weapons and rebels to enter Syria, while also serving as a refuge for people fleeing the conflict.

Dozens of people wounded in fighting in Qusayr - mostly rebels - flocked to Arsal last week as Syria’s army and fighters from the Lebanese Shiite movement Hezbollah overran the former insurgent bastion near the border.

In a new macabre episode, Sunni Islamist rebels celebrated the killing of some 60 Shiites, mostly pro-regime fighters, according to amateur video distributed by the Syrian Observatory for Human Rights.

“Look Shiites, this is how you will end up, you dogs,” cries one man shown in footage filmed in Hatlah village, in the eastern Syrian province of Deir Ezzor, as he points to a man’s disfigured corpse.

Boosted by their victory in Qusayr, regime troops advanced on parts of the nearby city of Homs, as they prepared to assault Aleppo in the north, the Observatory said.

A US-Russian peace initiative appears to have stalled as Assad’s forces make battlefield gains.

UN Secretary-General Ban Ki-moon, meanwhile, called for better protection for peacekeepers in the Golan Heights against the growing threat from the fighting in Syria but said it was “essential” that the mission remain.

Ban made the call as the first Austrian troops withdrew from the UN Disengagement Observer Force (UNDOF) following attacks and abductions of peacekeepers. Croatia and Japan have also withdrawn in recent months.

Fiji is sending 171 troops this month, who would replace the Croatian and Japanese contingents, according to the UN report. It has reportedly offered to send hundreds more soldiers.

The United Nations also said on Wednesday that children are being used as sniper targets and human shields in the Syria war.

Ban said in the report the 26-month-old war in Syria is taking an “unacceptable and unbearable” toll on children with thousands among the death toll.

UN leader Ban Ki-moon said in the report the 26-month-old war in Syria is taking an “unacceptable and unbearable” toll on children with thousands among the death toll.

The office of UN special representative on children in conflict, Leila Zerrougui said it had received “verified reports that Syrian children are killed or injured in indiscriminate bombings, shot by snipers, used as human shields or victims of terror tactics.”

Zerrougui presented the report which said boys as young as 10 are used by armed groups to work as combatants and porters.

It said there were a growing number of reports of the Free Syrian Army, the main rebel group, recruiting children, mostly between 15 and 17.

Sexual violence has been used by al-Assad’s forces against boys to obtain information or confessions, the special representative said.

Child detainees as young as 14 were tortured like adults “including electric shock, beatings, stress positions and threats and acts of sexual torture.”

Thousands of children “have seen family members killed or injured,” said the report.

“Everyone involved in the conflict needs to take urgent measures to protect children,” Zerrougui said.

“Allowing access for lifesaving humanitarian assistance is essential. We cannot allow innocent children to continue to die because they can’t see a doctor, or because they can’t fulfil their basic needs.”