The International Committee of the Red Cross (ICRC) says fighting in Syria is so widespread that the conflict is now in effect a civil war.
The change in status means combatants will now be officially subject to the Geneva Conventions, leaving them more exposed to war crimes prosecutions.
The Red Cross had previously regarded only the areas around Idlib, Homs and Hama as war zones.
Meanwhile, the Syrian government is disputing claims it used heavy weapons.
Activists initially described fighting on Thursday in the village of Tremseh near Hama as a massacre of dozens of civilians, but later accounts suggested most of the dead were armed rebels.
The UN accused Syrian forces of using heavy artillery, tanks and helicopters, but Damascus denied those allegations and said just two civilians had been killed.
The accusations, if proved, would mean Damascus had broken an agreement it made with envoy Kofi Annan.
Later on Sunday, video footage emerged purporting to show heavy fighting in southern Damascus.
Activists claimed the fighting was the most intense seen in the capital since the start of anti-government protests in March last year.
They said tanks and mortars were used, and in some areas residents were fleeing.
There has been no independent confirmation of their claims.
The International Committee of the Red Cross, which oversees the Geneva Conventions, said fighting had now spread beyond the three hotspots of Idlib, Homs and Hama.
Spokesman Hicham Hassan said Syria was now regarded as a "non-international armed conflict", which is the technical term for civil war.
"What matters is that international humanitarian law applies wherever hostilities between government forces and opposition groups are taking place across the country," he said.
The BBC's Imogen Foulkes in Geneva says the statement is significant because it is the Red Cross' job to monitor the conduct of the fighting, and to tell warring parties what their obligations are.
Under the Geneva Conventions, indiscriminate attacks on civilians, attacks on medical personnel or the destruction of basic services like water or electricity are forbidden and can be prosecuted as war crimes.
From now on, all those fighting in Syria are officially subject to the laws of war, and could end up at a war crimes tribunal if they disobey them.
Last month, the UN's head of peacekeeping Herve Ladsous also said Syria was in a state of civil war.
And Syrian President Bashar al-Assad has previously said the country is in a state of war.
Some 16,000 people are thought to have been killed since the uprising against Mr Assad's regime began in March 2011.
UN diplomats are attempting to agree a way forward for the organisation's monitoring mission in the country.
The mission's mandate runs out on Friday, and Western nations are trying to get Russia and China to agree to a beefed-up resolution authorising sanctions.