Opposition activists say at least 20 people have been killed in clashes between troops loyal to President Bashar al-Assad and rebels in southern and central Syria.
The London-based Syrian Observatory for Human Rights said four soldiers and 12 rebels were among those killed on May 21.
There was fierce fighting in southern Daraa Province and the central region of Hama.
Activists said government forces shelled the central province of Homs, killing four people and wounding more than 10.
UN Secretary-General Ban Ki-moon warned on May 21 that efforts for a solution to the Syrian crisis were at a "pivotal moment," and reiterated concerns about a full-blown civil war amid signs that a five-week-old truce was unraveling.
On May 20, NATO ruled out military action against Assad's regime.
Based on reporting by AFP and dpa