BEIRUT – Thirteen bound corpses, many apparently shot execution style, have been discovered in eastern Syria, U.N. observers said Wednesday, days after the massacre of more than 100 people provoked international outrage and the coordinated expulsion of Syrian diplomats from world capitals.

The latest killings happened in Deir el-Zour province, where the bodies were found late Tuesday blindfolded with their hands tied behind their backs. A statement by the U.N. mission said some appeared to have been shot in the head at close range.

A video posted online by activists showed the men lying face down, pools of dried blood under their heads.

The head of the U.N. observer team, Maj. Gen. Robert Mood, said he was “deeply disturbed by this appalling and inexcusable act.”

The fresh killings underline violence that seems to be spiraling out of control as the uprising against President Bashar Assad that began in March 2011 has morphed into an armed insurgency. Activists say as many as 13,000 people have been killed since the revolt began.

In the wake of last weekend’s massacre in Houla, in which nearly half of the 108 dead were children, the United States and Western nations expelled Syrian diplomats in protest – a move Syria’s state-run media denounced Wednesday as “unprecedented hysteria.”

The massacre drew continued harsh criticism Wednesday, even from Syria’s closest ally Iran, with Iranian President Mahmoud Ahmadinejad saying that anyone responsible for the killings should be punished. “I’m not excluding anyone from this responsibility,” Ahmadinejad told France 24 TV station.

U.N. investigators and survivors have blamed pro-regime gunmen for at least some of the carnage in Houla, a collection of poor farming villages in central Homs province, saying men in civilian clothes gunned down people in the streets and stabbed women and children in their homes. The Syrian government denied its troops were behind the killings and blamed “armed terrorists.”

Damascus 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. The U.N.’s top human rights body planned to hold a special session Friday to address the massacre.

Meanwhile, violence continued unabated. Syrian forces bombarded rebel-held areas and clashed with army defectors in Homs province, killing at least eight people, activists said.