Turkey and Japan expelled Syrian diplomats on Wednesday, joining the U.S. and several other nations in protesting a weekend massacre of more than 100 people in Syria, including women and children.

The move came as Syrian forces bombarded rebel-held areas in the same province where the Houla killings occurred, although no casualties were immediately reported, activists said.

Survivors blamed pro-regime gunmen for at least some of the carnage in Houla as the killings reverberated inside Syria and beyond, further isolating President Bashar Assad and embarrassing his few remaining allies. The Syrian government denied its troops were behind the killings and blamed "armed terrorists."

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

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 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, which began Tuesday with announcements by the United States, Britain, Canada, Australia, France, Germany, Italy, Spain and Bulgaria, 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 regime. 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.

The announcements came a day after the United States, Britain, Canada, Australia, France, Germany, Italy, Spain and Bulgaria ordered top Syrian diplomats to leave.

U.N. special envoy Kofi Annan met with Assad on Tuesday in Damascus to try to salvage what was left of his peace plan, which since being brokered six weeks ago has failed to stop any of the violence on the ground.

The Al-Baath daily, the mouthpiece of Assad's Baath Party, said Syria won't be intimidated by such "violent rhythms" and would remain standing in front of such "ugly, bloody and dramatic shows." It added that "Syria will not tremble as they think."

The government's Al-Thawra newspaper also blasted the Western decision, calling it an "escalation that aims to besiege Annan's plan and enflame a civil war."

Tensions have escalated as more information emerges about the May 25 killings in Houla.

The U.N.'s human rights office said most of the 108 victims were shot execution-style at close range, with fewer than 20 people cut down by regime shelling.

U.N. peacekeeping chief Herve Ladsous said there are strong suspicions that pro-Assad fighters were responsible for some of the killings, casting doubt on allegations that "third elements" _ or outside forces _ were involved, although he did not rule it out.

On Sunday, Foreign Ministry spokesman Jihad Makdissi said a committee comprising the ministries of justice, defense and interior was set up to investigate the massacre and would have the job done within three days.

Meanwhile, activists said Syrian troops shelled restive suburbs of Damascus and rebel-held areas in the central city of Homs on Wednesday.

The Britain-based Syrian Observatory for Human Rights and the Local Coordination Committees said at least five people were killed in the Damascus suburb of Douma. Both groups had no details about casualties in Homs, which is the provincial capital of the province that includes Houla.

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Mroue reported from Beirut. Associated Press writer Albert Aji contributed to this report from Damascus, Syria.