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BEIRUT — Syria's government and rebels on Wednesday both demanded an international investigation into an alleged chemical weapons attack, as the country's feared arsenal became the latest propaganda tool in the 2-year-old civil war.

The U.S. ambassador to Syria said the Obama administration has no evidence that chemical weapons were used, but he warned that an increasingly desperate President Bashar Assad might be tempted to use his stockpile.

The use of chemical weapons by either side is a nightmare scenario. Along with its warnings about Assad, the West is just as concerned that rebel forces, including some linked to al Qaeda, could get their hands on Syria's chemical weapons supplies.

Despite the importance, any clear confirmation of the nature of the attack that took place Tuesday in the northern village of Khan al-Assal, killing at least 31 people, is unlikely. Syria's government seals off areas it controls to journalists and outside observers.

The two sides blamed each other for a chemical attack without offering clear proof or documentation, as has frequently been the case in the Syrian civil war.

If confirmed, it would be the first time a chemical weapon has been used in Syria's war that has already killed an estimated 70,000 people.

Syria's UN Ambassador Bashar Jaafari told reporters at the United Nations Wednesday that he had asked UN Secretary-General Ban Ki-moon to form "a specialized, independent and neutral technical mission to investigate the use by the terrorist groups operating in Syria of chemical weapons" in Khan al-Assal.

Jaafari called the attack "very serious and alarming and unacceptable and unethical."

UN spokesman Martin Nesirky said he would have something to say "once we receive any formal request, which we have so far not received." He said the secretary-general remains convinced that the use of chemical weapons by any party under any circumstances would constitute "an outrageous crime."

Syria's main opposition group also demanded an international investigation.

"All evidence now indicates that the Assad regime is using these weapons against its own people," the main Western-backed opposition group, the Syrian National Coalition, said.

"The Coalition demands a full international investigation, and asks for a delegation to be sent to inquire and visit the site," the group said in a statement.

President Barack Obama has declared the use, deployment or transfer of the weapons to be his "red line" for possible military intervention in the Arab country.

The U.S. ambassador to Syria said Wednesday the Obama administration has no evidence to support President Bashar Assad's claims that U.S.-backed rebels used chemical weapons in northern Syria, but is looking carefully at the conflicting reports.

"So far we have no evidence to substantiate the reports that chemical weapons were used yesterday," Robert Ford told the House Foreign Affairs Committee. He later clarified that the administration was extremely concerned and was trying to verify reports that such weapons were used in the Aleppo province and the Damascus suburbs.

He warned that Assad might resort to using chemical weapons if he faces defeat.

Ford was withdrawn from Damascus in October 2011 and has not returned since.

Russia and Iran, Assad's main allies, backed his regime's charges.

Iranian Foreign Ministry spokesman Ramin Mehmanparast blamed "armed opposition groups," calling use of chemical weapons "an inhuman act."

"Undoubtedly, the responsibilities of a repetition of such crimes would fall on those committing it and the countries that support them," he was quoted by state TV as saying, apparently referring to Gulf states such as Qatar and Saudi Arabia

"There's definitely a propaganda war between the regime and opposition," said Ayham Kamel, a Middle East analyst at the Eurasia Group in London.

"Because we cannot verify either claims, we are going to be stuck in the same cycle of accusations, unless some international mission is actually sent there to verify what happened," he said.

Jordan's king warned Wednesday that an extremist Islamic state could form on his border.

King Abdullah II told The Associated Press in an interview that in his view, Assad was beyond rehabilitation, and it was only a matter of time before his authoritarian regime collapses.

"The most worrying factors in the Syrian conflict are the issues of chemical weapons, the steady flow or sudden surge in refugees and a jihadist state emerging out of the conflict," the king said.

The opposition's disunity was on display again Wednesday.

About a dozen members of the Syrian National Coalition suspended their membership a day after it elected the first rebel prime minister.

Among them were senior members including Suheir Atassi, Kamal Labwani, and spokesman Waleed al-Bunni.

Coalition member Mohammed Qaddah confirmed the suspensions. Asked whether it was related to divisions over the election of Ghassan Hitto, a little-known American-educated IT manger, to head an opposition interim government, he said "it was one of several reasons."

Atassi said she suspended her membership because there was a group in the coalition that seeks to control it, while the others are expected to follow. "I refuse to be a follower and I refuse to be simply a woman who decorates their gatherings and conferences while they make all the decisions," she wrote on her Facebook page.

Coalition members have complained of the dominance of the fundamentalist Muslim Brotherhood in the SNC, and Hitto was one of the top Brotherhood candidates.

Kamal Labwani said Hitto "was just a face" since he had no popular support in the country and suggested he was imposed by the Brotherhood and its Qatari backers.

Assad made a rare public appearance Wednesday, visiting a fine arts school in Damascus and meeting the parents of students who were killed in the civil war, state TV reported.

Photos run by the Syrian state media showed Assad shaking hands and listening closely to people who were said to be parents of war victims. It was his first appearance outside his palace since January, when he delivered a speech.

In Lebanon, the state-run National News Agency said five rockets fell on the Lebanese side of the border in the town of al-Qasr. It said there were no casualties.

On Monday, the NNA said Syrian warplanes hit targets along Syria's border with Lebanon. Lebanese President Michel Suleiman condemned the air raid. The Foreign Ministry in Damascus denied Syrian aircraft bombarded Lebanon.