Syrian troops are shelling the rebel-held neighbourhood of Khaldiyeh in the central city of Homs, according to activists.
The activists said the bombardment appears to be a preparation for the troops to storm the area, which has been in rebel hands for months.
The Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees had no immediate word on casualties.
Amateur videos posted online showed a small white plane, apparently a drone, flying over Homs.
The city has been one of the hardest hit regions in Syria since an uprising against president Bashar Assad's regime began in March last year.
Today's violence comes two days after reports of mass killing in the nearby province of Hama, where about 80 people, including women and children, were shot, stabbed or burned.