The international mediator on Syria said on Wednesday its civil war risks spilling across borders to engulf the Middle East and appealed for a temporary truce he said could mark a small step towards defusing 19 months of conflict.

Lakhdar Brahimi, the UN-Arab League envoy, has proposed that both President Bashar al-Assad’s forces and rebel fighters seeking his overthrow hold fire during the Islamic feast holiday of Eid al-Adha that starts next week.

Syrian authorities, who blame rebels for the failure of an April ceasefire plan, guardedly welcomed Brahimi’s proposal but said any initiative must be respected by both sides.

Turkey, one of Assad’s harshest critics, and Iran, one of his strongest allies, both backed the plan, in rare agreement. Thirty thousand people have been killed in the uprising, which began with peaceful demonstrations and now pits mainly Sunni Muslim rebels against an Alawite president. There are fears of broader Middle East sectarian conflict between Sunni powers sympathetic to the rebels and Shi’ites who back Assad. “This crisis cannot remain within Syrian borders indefinitely. Either it will be addressed or it will increase ... and be all-consuming,” Brahimi told reporters in Beirut after talks with Lebanese leaders.

The British-based Syrian Observatory for Human Rights said at least 90 people had been killed in Syria by late afternoon on Wednesday and 150 the day before. The death toll has topped 1,000 a week for at least two months as divided world powers have condemned the bloodshed in what has become a largely stalemated conflict.