Syria's powerful neighbor Turkey indicated that its patience was running thin with lack of progress in its efforts to convince Assad to halt a relentless military assaults to crush the popular uprising, during which a grassroots activists group said more than 3,000 civilians have been killed.

In a first direct call by the opposition for foreign intervention, Syria's underground opposition said a rise in the number of protesters killed during the revolt had won over many reluctant Syrians to the need for outside help.

The Syrian Revolution General Commission umbrella bloc of activists appealed to the international community on Thursday to send in human rights monitors to help deter military attacks on civilians in the increasingly bloody crackdown.

"Calling for outside intervention is a sensitive issue that could be used by the regime to label its opponents as traitors. We are calling for international observers as a first step," spokesman Ahmad al-Khatib told Reuters.

The announcement came as Syrian forces arrested dozens of people in house-to-house raids in the city of Homs following military operations that killed at least 27 civilians on Wednesday. Activists and residents also reported more defections among the rank-and-file army.