The UN's top observer said on Wednesday he was "deeply disturbed" by a newly discovered massacre in Syria, as the opposition insisted President Bashar al-Assad must step down if a peace plan is to be saved.

The discovery of the bodies of 13 people apparently killed execution-style came as Western powers raised anew the prospect of armed intervention in Syria to protect civilians, prompting cries of protest from traditional Damascus allies China and Russia.

Major General Robert Mood said the 13 bodies had been found on Tuesday night in the area of Assukar, 50 km (31 miles) east of Deir Ezzor.

"All the bodies had their hands tied behind their backs and some appear to have been shot in the head from a short distance," he said in a statement.

The veteran Norwegian peacekeeper said he was "deeply disturbed by this appalling and inexcusable act."

Mood's comment comes amid mounting international rage over a massacre at the weekend in which 108 people, mainly women and children, were killed in the area of Houla, in central Syria.

A substantial number of those killed also appeared to have been summarily executed, the United Nations has said.

The Assad regime has denied any involvement in the Houla killings, blaming them as it regularly does on "terrorists."

Britain, France, the United States and other Western nations have expelled Syrian diplomats in the wake of the Houla massacre while France floated the idea of armed intervention to protect civilians.

On Wednesday, Japan joined the chorus of international outrage, telling the Syrian ambassador in Tokyo to leave the country "as soon as possible," while Turkey ordered Syrian diplomats to leave the country within 72 hours.

French President Francois Hollande had said on Tuesday he did not rule out military intervention, provided it were approved by the UN Security Council.

"An armed intervention is not excluded on the condition that it is carried out with respect to international law, meaning after deliberation by the United Nations Security Council," he said.

Australia said it was open to discussing military intervention, but warned of the significant challenges involved in getting it off the ground.

Meanwhile, the opposition Syrian National Council said the only way for UN-Arab League envoy Kofi Annan's peace plan to be saved is for Assad to resign.

"An international understanding for Assad's stepping-down is the only way to save Annan's plan and the political solution; otherwise the situation is on the verge of explosion and will threaten the entire region," SNC chief Burhan Ghalioun said.

In a statement issued after a telephone conversation with German Foreign Minister Guido Westerwelle, Ghalioun stressed the "necessity that the international community moves quickly to put an end to the massacres."

Either the international community should "work with Russia to overcome the division in the Security Council, or with the Friends of Syria" group, the statement added.

China on Wednesday restated its opposition to armed intervention in Syria while Russia sought to halt fresh UN Security Council action.

"China opposes military intervention in Syria and opposes regime change by force," foreign ministry spokesman Liu Weimin told reporters in Beijing.

Liu added that China urged all parties to implement UN-Arab League envoy Kofi Annan's peace plan and to seek to end the crisis through negotiations.

With Russian and Chinese support, the UN Security Council on Sunday strongly condemned the Syrian government for using artillery in Houla.

But Russia, which along with China had vetoed two earlier UN Security Council resolutions highly critical of Assad's regime, on Wednesday said it was "premature" for the council to consider new action.

"We believe that a review now by the Security Council of any new measures on the situation would be premature," Deputy Foreign Minister Gennady Gatilov told the Interfax news agency.

In its first reaction to the diplomatic expulsions, Syria gave the Dutch charge d'affaires 72 hours to leave the country.

As part of the worldwide response on Tuesday, the Netherlands had declared Syria's ambassador as "persona non grata."

Moscow characterised as "counterproductive" expulsion of Syrian envoys.

"After all, vital (diplomatic) channels ... end up being closed," the Russian foreign ministry said.

Annan, during a meeting with Assad in Damascus on Tuesday, urged the Syrian leader to act immediately to end the bloodshed that has claimed thousands of lives, warning that the country had reached a "tipping point."

On Tuesday alone, according to the Syrian Observatory for Human Rights, a total of 98 people were killed across Syria with another nine dying violently on on Wednesday morning.

More than 13,000 people have been killed, most of them civilians, since the uprising against Assad's regime erupted in March last year, according to the Britain-based Observatory.