The cities of Homs and Idlib have experienced particularly heavy fighting during the 14-month old uprising against President Bashar al-Assad
Both hotspots have satisfied the ICRC's three qualifying criteria for non-international armed conflict: intensity, duration and the level of organization of rebels fighting government forces.
It can be a situation of internal armed conflict in certain areas, an example was the fighting in Baba Amr in Homs in February, Kellenberger told Reuters.
The definition means that the Geneva Conventions on humane treatment of prisoners are now applicable to both sides of the conflict in the parts of Syria affected.
While this requires the humane treatment of prisoners on both sides, it also allows for the attack of military targets.
The news comes as the Red Cross appealed for more funds for the ICRC operation in Syria, the organization's 8th-largest in the world.
Tens of thousands of men, women and children are still displaced, said Kellenberger. Some are hosted by resident communities, while others have found refuge in public buildings. We are striving to bring them the help they urgently need by stepping up our emergency humanitarian response.''
The ICRC is currently sending around 100,000 food parcels a month to particularly vulnerable Syrians, but needs to expand its nearly 38 million Swiss francs ($41 million) budget for this year, according to Reuters.
The number of people who have very basic needs in terms of food and non-food items has increased very much. It is not only [the] consequences of fighting, it partly has to do with isolation of country through sanctions, Kellenberger added.
One of our biggest problems is to ensure wounded and sick have access to medical care without being afraid of being ill-treated. One of our biggest concerns is medical staff not being respected, be it the Syrian Arab Red Crescent or doctors doing surgery in private residences, he said.