THE United Nations yesterday warned that "the eyes of the world" were watching Syria as government troops, supported by Hezbollah guerrillas, besieged a key border town and battled rebels.

The Britain-based Syrian Observatory for Human Rights said fighting was taking place inside Qusair and in villages around it, in an area largely controlled by President Bashar Assad's forces.

The UN warned all sides would be held accountable for the suffering of civilians trapped there.

Rebels have pleaded for military help and medical aid for the hundreds of people wounded in the onslaught by government forces.

Fierce fighting also continued around the capital, Damascus, and elsewhere in the country.

UN Secretary-General Ban Ki-moon called on both sides to allow civilians to escape the town.

Hezbollah, the powerful Lebanese Shia organisation, is helping Assad fight the uprising against his rule, a move which has heightened concerns over sectarianism in the region. Rebels fighting against Assad's regime are overwhelmingly from the rival Sunni group, with some factions linked to al-Qaeda.

Yesterday, 16 rockets fired from Syria struck the eastern Lebanon region of Baalbek, a stronghold of Hezbollah. The rockets caused a fire in fields without any casualties, security officials said.

Meanwhile, police raids carried out in Turkey's largest city, Istanbul, have highlighted growing concerns that Syria's civil war is spilling over into other neighbouring states.

Local media reported that 12 people from Syria's al-Qaeda-linked al-Nusra Front had been found with the nerve agent sarin gas and had been planning an attack in Turkey.

However, Turkish authorities later denied that sarin had been uncovered, saying only that "unknown chemical materials" had been found and would be investigated.

The find came as reports emerged that a British national killed in Syria was believed to have travelled to the country to fight the Assad regime. The man, named as Ali Almanasfi, 22, from London, is said to have been killed alongside an American woman and another Westerner while photographing military positions in Idlib province, near the Turkish border.

According to reports, Almanasfi had grown up in London but his Muslim family are from Damascus.

On Friday, the US and Germany warned Russia not to endanger a planned peace conference for Syria – backed by Russia and the US and due to be held in Geneva this month – or alter the balance of power in the Middle East by providing S-300 anti-aircraft missiles to Assad's regime.

■SYRIA