The head of the UN humanitarian affairs has said parties to the Syrian conflict should avoid civilian injury and loss of life or risk being charged with crimes against humanity.
In a statement released on Monday, Valerie Amos said the Syrian government and those fighting it should uphold their obligation to distinguish between civilians and combatants, adding that "directing attacks against civilians constitutes a war crime".
"The escalating conflict in Syria is having a devastating physical and psychological impact on hundreds of thousands of people," Amos said.
"As the International Committee of the Red Cross has now described the situation as an armed conflict, international humanitarian law applies across Syria in areas where there is fighting."
Amos said humanitarian agencies had managed to increase assistance in the last month despite the "very dangerous and complex operating environment".
In-depth coverage of escalating violence across Syria
"Food aid is now reaching 850,000 people, and more than 100,000 displaced people have received mattresses, blankets, kitchen sets and other vital supplies in the last four weeks," she said.
Amos's comments came as UN envoy Kofi Annan continued his diplomatic mission in Russia to try to resolve the 16-month conflict which has left at least 14,000 people dead, according to UN estimates.
Annan is due to meet Vladimir Putin, the Russian president, after meeting his foreign minister, Sergei Lavrov.
Russia and China have previously vetoed UN Security Council resolutions seeking to sanction the Syrian government for targeting civilians in its ongoing war with armed groups it calls "terrorists".
The Security Council will vote on Wednesday on a Western-backed resolution that threatens Syrian authorities with sanctions if they do not stop using heavy weapons in towns. Russia has declared it will block the move.
The resolution, proposed by Britain, the United States, France and Germany, would extend a UN observer mission in
Syria for 45 days and place international envoy Kofi Annan's peace plan under Chapter 7 of the UN Charter.
Chapter 7 allows the 15-member council to authorise actions ranging from diplomatic and economic sanctions to military
intervention. US officials have said they are talking about sanctions on Syria, not military intervention.
Also, Ban Ki-moon, the UN chief, is visiting China to try to seek support for tougher action against the regime of Bashar al-Assad.
But a leading Chinese newspaper warned against outside intervention in Syria even as Western powers are pushing for a new UN resolution.
"The life of Syria's current political leadership can only be determined by the Syrian people. This is an internal matter and the international community should respect that," the state-run People's Daily said in an editorial.
Ban will arrive in Beijing later on Tuesday for talks with Hu Jintao, the president, for talks that are expected to be dominated by Syria.
Ban has already urged China to use its influence to back a peace plan by Annan, who is calling on the Security Council to order "consequences" for any failure to carry out his six-point plan.
Assad has agreed to the plan, which includes the withdrawal of heavy weapons, but failed to carry it out.
"External intervention to achieve regime change and to prevent a humanitarian disaster may appear to be sensible and responsible reasons to act," said the People's Daily, the mouthpiece of China's Communist Party.
Meanwhile, Syrian Expatriates, a pressure group, said on Tuesday it had received a report from activists inside Hama, saying Syrian military had committed a new massacre in AlHamideah neighborhood in Hama.
Fighting continued for the third day in the capital, Damascus, as Syria's rebels announced the launch of a "Damascus volcano" offensive against regime forces, witnesses told the AFP news agency.
"Machinegun fire was heard in the Sabaa Bahrat square, and members of the security forces, armed with Kalashnikovs, ran across the square," site of the country's central bank, one witness said.
Heavy machinegun fire rocked in centre of the city, while shooting was also reported in Baghdad Street, a main road near the square. Two entry points into the square were briefly closed, and then reopened to traffic.
The shots were fired as fighters in anti-regime neighbourhoods of the capital clashed violently with the army, while regime forces deployed helicopters to fire into several districts.
A column of smoke could be seen on Tuesday billowing from the Midan district near the centre of Damascus, which has been pounded by regime troops since Monday.
"I can hear shelling, explosions and gunfire every once in a while," a Kfar Souseh resident told Al Jazeera in the early hours of Tuesday.
Troops deployed armoured vehicles near the historic neighbourhood of Midan, and activists said residents were fleeing nearby Tadamon.
The fighting briefly closed the highway linking the capital with Damascus International Airport on Monday.