The head of UN observers in Syria, Major-General Robert Mood, said 13 bodies had been discovered in the east of the country, with their hands tied behind their backs and signs that some had been shot in the head from close range.
The bodies were discovered late on Tuesday in the area of Assukar, 50km east of Deir al-Zor.
"General Mood is deeply disturbed by this appalling and inexcusable act," a statement issued by the observer mission said on Wednesday.
"He calls on all parties to exercise restraint and end the cycle of violence for the sake of Syria and the Syrian people."
The new violence came as Turkey and Japan became the latest countries to expel Syrian diplomats, joining 11 other nations in protesting against a weekend massacre of more than 108 people in Houla, including women and children.
Nevertheless, on Wednesday Syrian forces continued to bombard rebel-held areas in Homs, the main city of the central province where the Houla killings occurred, although no casualties were immediately reported, activists said.
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The Britain-based Syrian Observatory for Human Rights and the Local Co-ordination Committees - two opposition activist networks - said at least five people were killed in Douma, a Damascus suburb.
Survivors blamed pro-government armed men for most of the carnage in Houla as the killings created revulsion inside Syria and beyond, further isolating President Bashar al-Assad and embarrassing his few remaining allies.
The UN said most of the victims were shot execution-style at close range, with fewer than 20 people killed by regime
The Syrian government denied its troops were behind the killings and blamed "armed terrorists".
The UN's top human rights body plans to hold a special session on Friday to address the massacre.
Syria had said it would conclude its own investigation into the Houla deaths by Wednesday, but it was not clear if the findings would be made public.
Wednesday's UN observer report underlined how a peace plan drafted by UN-Arab League envoy Kofi Annan has failed to stem bloodshed or bring Syria's government and opposition to the negotiating table.
Annan's deputy Jean-Marie Guehenno told the Security Council that Syria's protesters "have lost fear and are unlikely to stop their movement", according to a diplomat with knowledge of the closed session.
Guehenno said direct engagement between government and opposition was "impossible at the moment" and expressed "serious doubts over the commitment of Syrian authorities to the Annan plan", the diplomat said.
The Houla killings prompted Western nations to expel Syrian diplomats in a co-ordinated protest, with the United States, Britain, Canada, Australia, France, Germany, Italy, Spain and Bulgaria ordering top Syrian diplomats to leave on Tuesday.
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Syria's state-run media on Wednesday denounced the diplomatic expulsions as "unprecedented hysteria".
Turkey, Syria's neighbour and a former close ally, joined the co-ordinated protest on Wednesday. Turkey has been among the most outspoken critics of the Assad government.
It closed its embassy in Damascus in March and withdrew the ambassador. Its consulate in Aleppo remains open.
The foreign ministry said it ordered the Syrian charge d'affaires and other diplomats at the Syrian embassy in Ankara to leave the country within 72 hours. The consulate in Istanbul will remain open for consular duties only.
"It is out of the question to remain silent and without any reaction in the face of this action, which amounts to a crime against humanity," the ministry said in a statement on Wednesday.
"This grave crime against humanity by those who have attempted a massacre of civilians cannot go unpunished."
Japan also ordered the Syrian ambassador in Tokyo to leave the country because of concerns about violence against civilians.
Japan's foreign minister, Koichiro Genba, said his country was not, however, breaking off diplomatic ties with Syria.
In reaction to the expulsion of Syrian diplomats, Syria ordered the Dutch charge d'affaires on Wednesday to leave the country, the foreign ministry said.
Iranian President Mahmoud Ahmadinejad said on Wednesday those behind the massacre in the Syrian town of Houla last week should be punished, and that the West could not be trusted to resolve the crisis as it wanted the removal of Assad.
"All those who carried out these murders are guilty and I hope the people responsible are punished," Ahmadinejad told France 24 television in an interview.
He said that the West and certain Arab countries were interfering in Syria. "We cannot trust these people, because their objective is to bring down Assad."
Russia said the "counterproductive" expulsion of Syrian envoys would only damage existing efforts to end the crisis through talks.
"The expulsion of Syrian diplomats from leading Western states seems to us to be counterproductive. After all, vital [diplomatic] channels ... end up being closed," the Russian foreign ministry said in a statement.
The US Treasury's economic sanctions chief said Wednesday that he will travel to Russia soon to discuss putting more pressure on Assad’s government.
David Cohen, Treasury Under Secretary for Terrorism and Financial Intelligence, said the US remains concerned over any possible financial links between Moscow and Damascus, as Washington and its allies in Europe and the Gulf seek to force Assad from power.
"I think we share with Russia the ultimate goal here, which is a ... transition with Assad no longer remaining in power," Cohen told reporters in Washington.
"We reacted swiftly along with our partners to send a message," Peter Wittig, Germany's ambassador to the UN, told Al Jazeera.
"My government is very strongly of the view that we must exhaust all possibilities … which is why we want to safeguard the Annan plan."
Source: Al Jazeera and agencies