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.
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 opposition-held areas in Homs, the main city of the central province where the Houla killings occurred, although no casualties were immediately reported, activists said.
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 the Douma, a Damascus suburb.
Both groups had no details about casualties in Homs.
Survivors blamed pro-government armed men for at least some 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's top human rights body plans to hold a special session on Friday to address the massacre.
Peace plan
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.
Diplomat expulsions
The Houla killings prompted Western nations to expel Syrian diplomats in a coordinated protest, with the United States, Britain, Canada, Australia, France, Germany, Italy, Spain and Bulgaria ordering top Syrian diplomats to leave on Tuesday.
Syria's state-run media on Wednesday denounced the diplomatic expulsions as "unprecedented hysteria".
Turkey, Syria's neighbor and a former close ally, joined the coordinated 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.
PHOTO CAPTION
Image from the Syrian opposition's Shaam News Network on May 30, shows UN monitors being watched by Syrian government soldiers in the town of Kfra Nubul.
Al-Jazeera