Amnesty International has released new satellite images showing the extent of heavy weapon use in and around the Syrian cities of Aleppo and Anadan.
The organisation has warned that both sides fighting in Aleppo may be held criminally accountable for their failures to protect the civilian population.
The satellite images show an increased use of heavy weaponry, including near residential areas.
Amnesty said they raise urgent concerns over the assault on the beleaguered city.
The Syrian army and rebel forces have been fighting for control of the country's largest city for more than three weeks.
The satellite images, which were analysed by Amnesty International USA and Digital Globe, were shot at the end of July in Aleppo and the nearby town of Anadan.
Some of the images reveal more than 600 probable artillery impact craters from heavy fighting between Syrian armed forces and armed opposition groups in Anadan.
An image from 31 July shows probable artillery impact craters next to what appears to be a residential housing complex in Anadan.
Amnesty is concerned that the deployment of heavy weaponry in residential areas around Aleppo will lead to further human rights abuses and breaches of international law.
The Syrian armed forces and members of armed opposition groups, such as the Free Syria Army, may be held criminally responsible if they fail to protect the civilian population caught up in the conflict.
As fighting intensifies in populated urban areas, more and more civilians are being exposed to danger, including unlawful killings, warned Amnesty.
Amnesty International USA Emergency Response Manager Christoph Koettl said: "Amnesty International is sending a clear message to both sides in the fighting: any attacks against civilians will be clearly documented so that those responsible can be held accountable.
"Turning Syria’s most populous city into a battlefield will have devastating consequences for civilians. The atrocities in Syria are mounting already.
"The Syrian military and the opposition fighters must both adhere to international humanitarian law, which strictly forbids the use of tactics and weapons that fail to distinguish between military and civilian targets."
Meanwhile, Iran has offered support to Syrian President Bashar al-Assad.
Saeed Jalili, head of Iran's Supreme National Security Council, said Iran would not let its close partnership with the Syrian leadership to be shaken by the uprising or external foes.
"Iran will not allow the axis of resistance, of which it considers Syria to be an essential part, to be broken in any way," Syrian television quoted Mr Jalili as saying.
Iran's Fars news agency said Mr Jalili told Mr Assad that Iran was prepared to provide humanitarian aid to Syria.