The Organization of Islamic Cooperation (OIC) has suspended Syria's membership in the 57-county Muslim bloc, citing President Bashar al-Assad's violent suppression of the country's revolt.
The move was agreed on the second day of an OIC summit in Mecca despite opposition by Iran.
In a final statement, the OIC leaders called for an immediate end to the violence in Syria and expressed "deep concern at the massacres and inhuman acts suffered by the Syrian people."
The United States welcomed the OIC decision, saying it sent a "strong signal" to the Assad regime.
It comes after at least 20 were reported killed in a government aerial bombardment of the town of Azaz, north of Aleppo.
Government troops and rebels have been fighting for control of Aleppo, Syria's commercial hub, for more than three weeks now.
Earlier, in Damascus, a bomb exploded at a military building and also near a hotel housing some 100 UN observers.
Three people were wounded in the blast claimed by the rebel Free Syrian Army.
The conflict also appeared to be spilling across the border into Lebanon, where armed Shi'ite tribesmen announced they had kidnapped more than 20 Syrians and a Turkish national to secure the release of a relative seized by rebels in Syria earlier in the week.
The kidnappings prompted the United Arab Emirates, Saudi Arabia, and Qatar to urge their citizens not to travel to Lebanon.
In Geneva, the UN said war crimes had been committed by both sides in Syria, with government forces or its allies responsible for most of them.
A UN investigation also blamed a pro-government militia for a May bloodbath in the village of Houla that left more than 100 civilians dead, nearly half of them children.
Meanwhile, UN humanitarian chief Valerie Amos met in Damascus with Syrian Foreign Minister Walid al-Moallem to discuss the worsening humanitarian situation in the country.
The Syrian Foreign Ministry said both sides had exchanged ideas to ensure humanitarian access to all troubled areas in the country.
With reporting by AP, Reuters, and dpa