The International Committee of the Red Cross (ICRC) has formally declared the Syrian conflict a full-scale civil war - a decision that could expose its protagonists to war crimes trials.

The ICRC had previously maintained that the fighting was confined to three major hot spots - Homs, Hama and Idlib but now says the struggle has escalated into a "non-international armed conflict", a technical term for a civil war.

International law will now apply to all conflict across the country, and combatants will be subject to the terms of the Geneva Convention, which prohibits indiscriminate attacks on civilians and medical personnel, as well as the destruction of basic amenities such as water and electricity.

Combatants who breach the terms of the Geneva Convention could be subjected to a war crimes tribunal under United Nations (UN) jurisdiction.

The declaration of civil war comes just days after forces loyal to President Bashar al-Assad attacked the town of Tremseh in Hama province, reportedly killing over 100 people.

UN peace monitors visited Tremseh on Sunday to hear testimonies from local residents, who reported that pro-government troops and militia began their attack by shelling the town, before burning houses and carrying out summary executions.

Meanwhile, heavy fighting between Syrian forces and rebels continued in Damascus into the early hours of Monday, according to reports quoting residents and activists.

Loud explosions, continuous gunfire and sirens were heard overnight, Reuters reported. Residents described it as the worst fighting so far in the 17-month uprising against President Assad.

"I can't believe it, it sounds incredibly close. I hear shooting and other stuff, like blasts. I can hear the sounds of ambulances rushing past. I am so afraid. People may die tonight," one of the residents close to the area of fighting told Reuters.

A live internet video stream showed thick black smoke rising from Damascus. According to the activists, fighting was particularly fierce in al-Tadamon, a district in the southern part of Damascus.

"There is the sound of heavy gunfire. And there is smoke rising from the area. There are already some wounded and residents are trying to flee the area," activist Samir al-Shami told Reuters over Skype.

"There are also armoured vehicles heading towards the southern part of the neighbourhood," he added.

UN-Arab League special envoy Kofi Annan will meet Russia's Vladimir Putin in Moscow on Tuesday to discuss the situation inside Syria, and urge him to help end the conflict.