Several Syrian rebel groups have announced a new offensive against the government in a move they said was a response to ceasefire violations from the forces of President Bashar al-Assad.

The groups, which included factions fighting under the banner of the Free Syrian Army and Ahrar al-Sham, said in a joint statement on Monday that they would respond with force to any army units that fired on civilians in what they called a fresh "battle".

The statement was sent to the Reuters news agency by Mohamed Rasheed, a spokesman for the Jaish al-Nasr rebel group.

It said the groups would set up a joint operations room and gave no further details about where any fighting might take place.

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A ceasefire deal in place since February has been strained to breaking point, particularly around the divided city of Aleppo, with each side blaming the other for an escalation that has underlined the huge challenge facing peace talks that are currently being held in Geneva.

Heavy air strikes were also reported north of Homs on Monday, killing at least four people, the UK-based Syrian Observatory for Human Rights said.

The main opposition bloc in Geneva, the High Negotiations Committee (HNC), accused the government of sending a message that it did not want a political solution, but a military solution that the opposition said would destroy the country.

Mohammed Alloush, the chief negotiator for HNC, said on Monday that accepting Assad as part of any transitional government, a longtime sticking point at the talks, was out of the question.

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Asaad Zoubi, also from the HNC, said opposition forces should respond to any government attack.

Zoubi reiterated calls for the release of people from government prisons, particularly children and women.

"We will not accept or negotiate unless we get what we want and our demands are met," Zoubi said.

The HNC said last week that it was willing to share membership of a transitional governing body with current members of the government, but not with Assad himself.

UN mediator Staffan De Mistura has said a political transition will be the main focus of the current round of talks, which aim to end a five-year war that has killed more than 250,000 people and forced millions to flee the country.