Fighters from as many as 29 countries have filtered into Syria to join a civil war that has split along sectarian lines, increasingly pitting the ruling Alawite community against the majority Sunni Muslims, UN human rights investigators said today.

The deepened sectarian divisions in Syria may diminish prospects for post-conflict reconciliation even if President Bashar al-Assad is toppled. And the influx of foreign fighters raises the risk of the war spilling into neighbouring countries.

“As battles between government forces and anti-government armed groups approach the end of their second year, the conflict has become overtly sectarian in nature,” the investigators led by Brazilian expert Paulo Pinheiro said in an updated report.

As a result, more civilians were seeking to arm themselves in the 21-month conflict. “What we found in the last few months is that the minorities that tried to stay away from the conflict have begun arming themselves,” said Karen Abuzayd, a member of the group.

Syrian government forces had increasingly resorted to aerial bombardments, including shelling of hospitals, and evidence suggests that such attacks are “disproportionate”, the report said. The conduct of hostilities by both sides is “increasingly in breach of international law,” it added.

Most of the foreign fighters slipping into Syria to join rebel groups, or fight alongside them, are Sunnis from other countries in the Middle East and North Africa, the UN investigators found. “They come from all over, Europe and America, and especially the neighbouring countries,” said Ms Abuzayd.

It said the Lebanese Shia group, Hezbollah, had confirmed that group members were in Syria fighting on behalf of Assad.

The UN report also cited reports of Iraqi Shia coming to fight and said Iran, a close ally of Assad, confirmed in September that its Revolutionary Guards were in Syria providing assistance. Tehran has denied military involvement in Syria.