Fears Western intervention in Syria may spark a wider regional war were raised on Monday after Hezbollah warned it would retaliate against Israel in the event of an attack.

The Lebanese Shia group said it would respond to a US attack with missile strikes of its own, directed at Israel. The group said it would launch the retaliatory strike from within Syria – where its fighters have been supporting the Syrian army in its fight against rebels for the past year – in order to shield Lebanon from becoming embroiled in the conflict.

“Hezbollah is controlling 8,000km in Homs and will not hesitate to participate in an attack by firing surface-to-surface missiles from Syria,” a source from the joint operations between Hezbollah and Syrian forces said in comments published in the Kuwaiti al-Rai newspaper.

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The warning came as United Nations official estimated five million Syrians have been displaced inside the country. In addition, two million Syrians have fled to neighbouring countries, according to UN figures. The total, about seven million, amounts to nearly one-third of Syria’s population. The funding gaps for the displaced remain wide, with donor countries sending less than one-third of the money needed to help those displaced.