Syria is in a state of all-out civil war and all sides must respect international humanitarian law or risk facing war crimes prosecutions, the Red Cross said on Monday.
The International Committee of the Red Cross had previously regarded only the areas around Idlib, Homs and Hama as zones of non-international armed conflict -- the diplomatic term for civil war.
"This is not a new development; we already notified those involved (in the fighting) and in April we said that this situation of non-international armed conflict applied to three specific areas," spokesman Alexis Heeb told AFP.
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The Geneva Conventions define how civilians should be protected during conflict and also the conditions for "people who are no longer fighting, for example those who are injured, and also the conditions in which they can be detained", added the spokesman.
This change in status means combatants could now be more exposed to war crimes prosecutions.
The ICRC has regularly asked the Syrian authorities to respect the rights of injured people in medical centres and to provide access to prisoners in various detention centres.
The Geneva-based aid agency is the only international organisation operating in Syria, in conjunction with the Syrian Arab Red Crescent.
Since March 2011, the Red Cross has only managed to visit prisoners on two occasions, in Damascus and Aleppo's main prison.
"For the moment we do not have access to the opposition (fighters') detention camps," ICRC president Jakob Kellenberger said last month, adding that it was "important" for this to happen.