Syria's government and rebels traded accusations Tuesday of a chemical attack on a northern village for the first time in the civil war, although the U.S. said there was no evidence it had happened.

The use of such weapons would be a nightmare scenario in the two-year-old conflict that has killed an estimated 70,000 people, and the competing claims showed awillingness by both sides to go to new levels to seek support from world powers.

One of the international community's biggest concerns is that Syria's arsenal of chemical weapons could be used by one side or the other, or could fall into the hands of foreign jihadi fighters among the rebels or the Lebanese militant group Hezbollah, which is allied with the regime of President Bashar Assad.

U.S. President Barack Obama has declared the use, deployment or transfer of the weapons would be a "red line" for possible military intervention by the U.S. in the Syrian conflict.

The accusations emerged only a few hours after the Syrian opposition elected a prime minister to head an interim government that would rule areas seized by rebel forces from the Assad regime.

31 killed in attack, Syrian official says

The state-run SANA news agency said "a missile containing a chemical substance" was fired at the village of Khan al-Assal in Aleppo province by "terrorists" — the term it uses for rebels. Deputy Foreign Minister Faysal Mekdad said 31 people were killed.

SANA added that more than 100 others were wounded, some of them critically, and it published pictures showing casualties, including children, on stretchers in what appears to be a hospital ward. None showed signs of physical injuries.

"It is another crime to be added to the record of armed terrorist groups that are supported by some Arab countries and Western countries," Mekdad told reporters in Damascus.

To back up its claims of a chemical attack by the rebels, SANA pointed to videos posted on YouTube several months ago that purported to show regime opponents experimenting with poisons on mice and rabbits. The origin of the videos was not known.

The rebels quickly denied using chemical weapons and accused regime forces of doing so.

Maj. Gen. Adnan Sillu, who was previously among those in charge of Syria's chemical weapons training program before he defected to Turkey last year, accused the Assad government of firing a chemical weapon. "Only the regime has long range missiles capable of handling chemical agents," he said in comments to Arab broadcaster Al Arabiya.

The head of Syria's main opposition group, the Syrian National Council, said the group was still investigating the attack.

The Britain-based Syrian Observatory for Human Rights, which relies on a network of activists in the country, said a rocket attack on Khan al-Assal killed at least 26 people. The group's director, Rami Abdul-Rahman, said he had no information that chemical weapons were involved and said the rocket landed near a military installation.

The village lies just west of the city of Aleppo and had seen fierce fighting for weeks before rebels took over a sprawling government complex there last month. The facility included several military posts and a police academy that Assad's forces have turned into a military base that regularly shells nearby villages.

The Aleppo Media Center, affiliated with the rebels, said there were cases of "suffocation and poison" among civilians in Khan al-Assal after a missile was fired at the area. It said in a statement the cases were "most likely" caused by regime forces' use of "poisonous gases."

Mohammed al-Khatib, an activist in Aleppo, said regime forces meant to target rebels but the missile landed in a government-controlled area of Khan al-Assal instead.

"It caused a huge explosion, like a Scud missile does," he said. "Everyone knows rebels only have primitive rockets. Claims that opposition fighters were behind the attack are laughable."

U.S. 'deeply skeptical' of Assad regime's claims

A U.S. official said there was no evidence either side had used chemical weapons.

The origin of the attack is still unclear, the official said, adding that the Organization for the Prohibition of Chemical Weapons also was reporting no independent information of chemical weapons use. The official wasn't authorized to speak publicly on the matter and spoke on condition of anonymity.

The White House and the State Department rejected only the Assad regime's charge.

White House spokesman Jay Carney said the U.S. is looking carefully at all allegations, but said the Obama administration is "deeply skeptical" of any claims by the Assad regime.

"This is an issue that has been made very clear by the president to be of great concern to us," Carney said, adding that if the Syrian regime does use such weapons, "there will be consequences."

Russia, which has steadfastly supported Assad, backed the Syrian government's assertion. The Russian Foreign Ministry said the rebels detonated munitions containing an unidentified chemical agent, but didn't give further details, and added that it was an "extremely dangerous" development.

Ahmet Uzumcu, director general of the OPCW, said he is "deeply concerned" about allegations of chemical weapons use and that the group will continue to monitor the situation closely.

U.N. Secretary-General Ban Ki-moon, who spoke to Uzumcu, "remains convinced that the use of chemical weapons by any party under any circumstances would constitute an outrageous crime," U.N. spokesman Martin Nesirky said.

The Syrian regime is believed to possess substantial stockpiles of mustard gas and a range of nerve agents, including sarin, a highly toxic substance that can suffocate its victims by paralyzing muscles around their lungs.

Syria's policy has been not to confirm or deny if it has chemical weapons. But in July, then-Foreign Ministry spokesman Jihad Makdissi told a news conference that Syria would only use chemical or biological weapons in case of foreign attack, not against its own people. The ministry then tried to blur the issue, saying it had never acknowledged having such weapons.

The Assad regime has not said that rebels have been able to seize any chemical weapons, "so we assume that the opposition does not possess such weapons," said Mustafa Alani, an analyst with the Gulf Research center in Geneva.

"I would not rule out that the military would use chemical weapons and try to pin it on the rebels," Alani said.

U.S. Secretary of State John Kerry said recently that the longer the war goes on, the greater the danger of its institutions collapsing and extremists getting their hands on chemical weapons.

In Damascus, meanwhile, residents and the SANA news agency said several mortar shells landed in a park behind the Saudi Embassy in the upscale Abu Rummaneh district. SANA said three people were killed and 13 were wounded by the attack that damaged several apartments, cars and shops.

Earlier Tuesday, Syria's opposition coalition elected Ghassan Hitto, a little-known American-educated technology manager to head an interim government to administer areas seized by rebels from Assad's troops.

Syria's provisional prime minister Ghassan Hitto, chosen by opposition leaders, condemned the alleged chemical weapons attack during a March 19 news conference in Istanbul, Turkey. (Osman Orsal/Reuters)

In his first speech after his election, Hitto ruled out dialogue with the regime.

"We confirm to our people that there is no place for dialogue with the Assad regime," he told members of the opposition Syrian National Coalition in Istanbul.

He said the interim government will be headquartered in rebel-held territories in northern Syria and urged international recognition for the new entity.

Mouaz al-Khatib, head of the Syrian National Coalition, said in Istanbul that the prime minister would develop his own program, first by trying to organize the revolutionary local councils established inside Syria.

He acknowledged that the new government faced challenges, adding that "the country has collapsed."

"The Syrian people have reached a stage of oppression, humiliation and destruction of infrastructure, so any positive effort now will present very positive steps," he said.