Observers from the UN have confirmed the discovery of 13 shooting victims in Syria as international outrage continues after the Houla massacre.
The men's bodies were found near the eastern city of Deir al-Zour, with their hands tied behind their backs, some apparently shot in the head.
UN mission chief Maj Gen Robert Mood said he was "deeply disturbed" by the "appalling and inexcusable act".
Hopes of a new initiative over Syria have been dented by Russia and China.
A Russian minister ruled out any action by the UN Security Council soon, saying Moscow "categorically" opposed foreign intervention.
The US ambassador to the UN, Susan Rice, has said a further escalation of the conflict in Syria is the likeliest scenario now, with it spreading to other countries in the region. She was speaking after a closed meeting of the UN Security Council in New York.
Her British counterpart, Mark Lyall-Grant, said that a third element, including jihadists, was now active in Syria. He called them "spoilers".
Turkey is expelling all Syrian diplomatic staff from Ankara, a day after a number of Western states announced a co-ordinated series of expulsions in response to the killing of 108 people in Houla last week.
As many as 15,000 people have been killed since the revolt against the rule of President Bashar al-Assad began in March of last year.
One of the organisations monitoring the violence, the UK-based Syrian Observatory for Human Rights, reported 39 new deaths in attacks across Syria on Wednesday. The figure, quoted by AFP news agency, included 15 government soldiers.
Gen Mood said the remains of the 13 men had been discovered on Tuesday night in al-Sukar, east of Deir al-Zour.
"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.
He called on all parties to the Syrian conflict to "exercise restraint and end the cycle of violence for the sake of Syria and the Syrian people".
China and Russia, which both wield a veto at the UN Security Council, have blocked two previous resolutions calling for tougher action on Damascus.
Russian Deputy Foreign Minister Gennady Gatilov said Moscow was "categorically against any outside interference in the Syrian conflict" because it would "only exacerbate the situation for both Syria and the region as a whole".
A spokesman for China's foreign ministry reiterated Beijing's opposition to military intervention in Syria or forced "regime change".
Russia's foreign ministry added in a statement that the expulsion of Syrian diplomats from leading Western states seemed to be "counterproductive". "After all, vital [diplomatic] channels... end up being closed," it added.
Turkey's move goes further than Tuesday's co-ordinated action by other states, which restricted their expulsions to the most senior staff. However, Syrian consular staff in Istanbul will remain.
In another development, Syria's honorary consul in California, Hazem Chehabi, resigned, saying he could not continue in his post after the "barbaric" massacre in Houla.
Children and women
Troops, tanks and artillery were all supposed to have been withdrawn to barracks in April as the first step on the ground under the six-point peace plan of UN and Arab League envoy Kofi Annan.
But it simply has not happened, says the BBC's Jim Muir in neighbouring Lebanon.
Amid the reports of continuing violence, it appears Mr Annan's talks with President Bashar al-Assad and opposition figures in Damascus on Tuesday have had no immediate impact on the ground.
Mr Annan's deputy, Jean-Marie Guehenno, told the UN Security Council on Wednesday that direct talks between the Syrian opposition and the government were currently "impossible", according to a diplomat who spoke to Reuters news agency on condition of anonymity.
Colonel Qassem Saadeddine, a rebel spokesman said to be currently inside the province of Homs, told AFP the government "only [understood] violence".
Forty-nine children and 34 women were among those killed in Houla, near Homs, last Friday and Saturday.
Witnesses have told the UN the vast majority of killings were committed by pro-government shabiha militiamen.
The UN Human Rights Council will hold a special session on Syria on Friday looking into the killings, officials said.