(CNN) -- Syria will complete its official probe Thursday into a massacre in Houla last week, the country's U.N. ambassador said Wednesday.
Syrian officials have repeatedly rejected government involvement in the massacre that left more than 100 people dead, about half of them children.
"The national commission of investigation in Syria will terminate its investigation tomorrow or after tomorrow," Bashar Jaafari told reporters at the United Nations. "And then you will hear, all of you ... the results of this investigation. And all of us will know for sure the identity of the perpetrators."
Jaafari's legal adviser told CNN that Syria expects to have results of its investigation Thursday.
Rupert Colville, spokesman for the U.N. human rights office, announced Tuesday that it's "clear" government forces were involved.
Most died as a result of "summary executions" by "armed men going into houses and killing men, women and children inside," Colville said, saying the Shabiha militia, a government militia group, appeared to be behind the executions.
Susan Rice, the U.S. ambassador to the United Nations, said the United States also believes it is "not ambiguous" that the masssacre was carried out "by every indication by the Shabiha militias acting on behalf of the government. We think it's quite clear-cut, and we think there needs to be justice and accountability for those who committed these atrocities."
Throughout the conflict in Syria, President Bashar al-Assad's regime has blamed violence on "armed terrorist groups," a line Jaafari continued Wednesday.
Those behind the "heinous crime," which was "unjustified and unjustifiable," will be brought to justice, Jaafari insisted. He also argued that countries are harboring and supporting such terrorist groups that are smuggling weapons into Syria.
The remarks came after a meeting of the U.N. Security Council, in which countries looked for new ways to pressure the regime amid growing international fury over the massacre and the growing death toll in the country.
Rice said after the meeting, "We certainly agree with Kofi Annan that this is a moment where we have reached, in effect, a tipping point."
Annan, special envoy to Syria for the United Nations and the Arab League, who has pushed a six-point peace plan, used the term "tipping point" Tuesday.
"I think we may be beginning to see the wheels coming off of this bus," Rice said. "And that means that what happens next and the steps that are taken by the Syrian authorities and by this council could well be dispositive."
"The political process which is so crucial to the success of any transition, which is the purpose of the Annan plan, is thwarted by the ongoing, escalating, expanding violence perpetrated by the government, and the reality that the opposition cannot possibly be expected to come to the table while the violence is intensifying, escalating and the government is lying about it," she said.
The United States and several other nations -- the Netherlands, Australia, Britain, France, Germany, Italy, Japan, Spain, Bulgaria, and Canada -- announced Tuesday that they were expelling some Syrian diplomats. Turkey joined that list Wednesday, expelling the charge d'affaires in Ankara and all the other diplomatic personnel at the embassy.
Syria said Wednesday it was expelling the Dutch charge d'affaires in Damascus, giving her 72 hours to leave the country.
Russia called the expulsions of Syrian diplomats "counterproductive" and insisted that a U.N. Security Council statement Sunday condemning the incident was for now "a strong enough signal to the Syria parties."
A central question hanging over the Security Council meeting is what steps Russia and China will allow. As permanent members, they have veto power, which they have used previously to block two resolutions. Many world leaders assailed the two nations, saying they were preventing steps that could stop the violence. But the two countries said they were seeking more balanced resolutions.
Both have major trade deals with Syria.
While no nation is openly pushing for military action, French President Francois Hollande is among those who have said it cannot be ruled out.
China and Russia spoke out forcefully Wednesday against such an idea.
"China opposes military intervention and does not support forced regime change," said Liu Weimin, a Chinese Foreign Ministry spokesman. "The fundamental route to resolving the Syrian issue is still for all sides to fully support (Kofi) Annan's mediation efforts and push all the relevant parties to carry out diplomatic dialogue."
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"One cannot take decisions on military operations in Syria by being guided by only emotions," said Russian First Deputy Foreign Minister Andrei Denisov, according to Russian news agency Itar-Tass.
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Denisov also said "the Russian position is not formed on the basis of emotions, which our respected French partners have unfortunately not escaped in the formulation of their position," according to Russian news outlet RIA Novosti.
The United States announced Wednesday it was sanctioning a Syrian bank that al-Assad's regime was using to circumvent other sanctions. The U.S. Treasury said its action will help isolate the regime "from the international financial system."
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Peter Wittig, Germany's U.N. ambassador, said he hoped the Houla massacre was "an eye-opener for some members of the council."
He said he hoped the council would consider whether to expand the U.N. monitoring mission in Syria. Also, he asked, "how does the council deal with violations of its own resolutions? Because the massacre in Houla was a clear violation of the Security Council resolution."
And, Wittig said, the massacre made clear that "we need mechanisms of accountability for those crimes. We think the last 15 months have shown that inaction of the council leads to this vicious circle of violence. We need to overcome that, and hopefully today's debate will make a contribution."
Speaking to reporters Wednesday, he added, "We have to exhaust all means to find a political solution," and "militarization, for us, is not an option."
The uprising in Syria began nearly 15 months ago. Syrian forces have engaged in a brutal crackdown on the opposition.
At least 13 people were killed Wednesday, including five in Douma, the opposition Local Coordination Committees of Syria said. The LCC also said regime forces launched tear-gas grenades at Aleppo University students and made arrests.
Meanwhile, the head of the U.N. observer mission said 13 bodies were discovered Tuesday night in the area of Assukar, 30 miles east of Deir Ezzor in the eastern part of the country.
"All the bodies had their hands tied behind their backs and some appear to have been shot in the head from a short distance," according to a statement from the office of Maj. Gen. Robert Mood.
Mood called the act "appalling and inexcusable" and urged "all parties to exercise restraint and end the cycle of violence."
Syria, on state-run news agency SANA, said 25 "army, law enforcement and civilian martyrs" were buried Wednesday. Also, an "armed terrorist group" attacked an oil pipeline in Deir Ezzor, SANA reported.
CNN cannot confirm death tolls and reports of violence from Syria because the government limits access by foreign journalists.
Jaafari insisted Syria supports Annan's six-point plan to halt the violence in Syria, and that his government is only acting against terrorist groups.
Annan met with the prime minister and foreign minister of neighboring Jordan about the Syrian crisis, his spokesman said.
"It is important to find a solution that will lead to a democratic transition in Syria and find a way of ending the killings as soon as possible. And in that context, I am very grateful for the support I am receiving from the government of Jordan and other governments around the world. With goodwill and hard work, we can succeed," Annan said.
His deputy, Jean-Marie Guehenno, addressed the Security Council on Wednesday.
"I told the council that the parties need to recommit to a full cessation of violence," Guehenno said to reporters after. "We have seen that this cessation of violence is under threat. Today, as the stronger partner, the government needs to take steps to this end, but it is also vital that the armed opposition be ready to adhere to a full cessation of violence."