The ongoing Syrian conflict and the impending US strike could snowball into another world war, warns the Vatican.

As Syrian President Bashar al-Assad warned any external intervention could trigger a regional war, Monsignor Mario Toso, the chief of the Vatican's Pontifical Council for Justice and Peace, said the conflict "has all the ingredients to explode into a war of global dimensions.

"The solution to Syria's problems is not in armed intervention. Violence will not decrease and there is a risk of a conflagration that extends to other countries," Toso told Vatican radio.

Middle East experts believe Barack Obama's determination to attack Syria could have severe ramifications in neighbouring Israel and Syria, while any strike would widen the rift with Russia, Assad's key ally.

In light of the heightened tensions, Pope Francis has called for a worldwide day of fasting and prayer for peace in Syria on 7 September.

"My heart is deeply wounded by what is happening in Syria and anguished by the dramatic developments. With utmost firmness, I condemn the use of chemical weapons. I tell you that those terrible images from recent days are burned into my mind and heart," said the pontiff.

Refugee Crisis

Meanwhile the UN's refugee agency has revealed that the number of refugees fleeing Syria has reached two million, with almost half of them leaving in the past six months alone.

More than 700,000 Syrians have sought shelter in Lebanon, the smallest of Syria's neighbours, according to a statement from the UN High Commissioner for Refugees.

The UNHCR said Syria "is haemorrhaging women, children and men who cross borders often with little more than the clothes on their backs," and appealed for "massive international support" in managing the refugee crisis.

Apart from the exodus, more than 110,000 people have been killed in Syria, since the uprising began in March 2011.