UN humanitarian chief Valerie Amos headed for Syria on Wednesday to urge the regime to allow aid into battered protest cities, as US President Barack Obama insisted military intervention would be a "mistake".

A Chinese envoy sent to discuss ways to end the bloodshed in Syria, meanwhile, was to discuss a six-point peace plan with Foreign Minister Walid Muallem and opposition figures, a newspaper reported.

Diplomats in New York said there would be no moves at the United Nations until UN-Arab League special envoy Kofi Annan and UN humanitarian chief Amos had also wound up separate missions to Syria this week.

Amos was to make a two-day visit, following widespread UN and Security Council complaints about President Bashar al-Assad's refusal to let her in, while Annan is due in Damascus on Saturday.

The five major UN powers discussed on Tuesday new efforts to press for a halt to the violence in Syria, which Obama called heartbreaking, as regime forces pounded rebel towns and the death toll rose.

However, Obama cautioned against unilateral military action.

The president was speaking after top Republican Senator John McCain called for US air strikes on Syrian forces to protect population centres and create safe havens for regime opponents.

"What's happening in Syria is heartbreaking, and outrageous, and what you've seen is the international community mobilise against the Assad regime," Obama told a White House press conference.

"On the other hand, for us to take military action, unilaterally, as some have suggested, or to think that somehow there's some simple solution, I think is a mistake."

The United Nations says the regime's crackdown has already cost more than 7,500 lives in the past year, but Assad on Tuesday vowed to press ahead with his campaign to crush "terrorism."

The United States is leading work on a text for the badly divided UN Security Council, where Russia and China have twice used their powers as permanent members to veto Syria resolutions.

A new draft obtained by AFP on Tuesday calls on the Syrian government to immediately cease all violence, withdraw security forces from protest cities and release prisoners held over the protests.

It then calls on the opposition to "refrain from all violence" once these conditions are met.

Ambassadors from permanent members Britain, China, France, Russia and the United States held talks along with the envoy from Morocco, the current Arab member of the council.

Russia's Deputy Foreign Minister Gennady Gatilov earlier said that his country, an ally of Damascus, believed the draft was still not balanced.

In Damascus, Chinese envoy Li Huaxin, quoted in Al-Watan daily, said he already met on Tuesday with Deputy Foreign Minister Ahmad Arnus to discuss his country's "six-point vision" on the year-old bloodshed in Syria.

China's foreign ministry had on Sunday unveiled the initiative, which calls for an immediate end to the violence and for dialogue between Assad's regime and the opposition.

Beijing's proposal rejects foreign interference or "external action for regime change" in Syria but supports the role of the Security Council "in strict accordance with the purposes and principles of the UN charter."

Li was also expected to meet representatives of opposition groups, according to Al-Watan.

On Tuesday, at least 16 people were killed as Syrian regime forces launched a major assault on Herak, a town in the southern province of Daraa, a monitoring group said.

"Large military forces, including tanks and armoured troop carriers, launched an assault on Herak," the Syrian Observatory for Human Rights added.

The International Committee of the Red Cross, meanwhile, has been negotiating with authorities since last week to be allowed to deliver aid and evacuate the wounded from the battered Baba Amr district of Homs city in central Syria.