Syrian state media said Tuesday that rebels fighting against President Bashar al-Assad used a chemical weapon in northern Syria, killing 25 people.

Syrian Information Minister al-Zoubi claimed in a statement that rebels had fired a rocket containing chemical agents. The Russian Foreign Ministry also said Tuesday that Syrian rebels had killed 16 people near Aleppo with chemical agents, the Associated Press reported.

"Terrorists launched a missile containing chemical products into the region of Khan al-Assal in the province of Aleppo," according to the BBC.

A senior rebel commander denied the government's accusation, countering that it was Assad's forces who had used chemical weapons.

"We were hearing reports from early this morning about a regime attack on Khan al-Assal, and we believe they fired a Scud with chemical agents," Qassim Saadeddine told Reuters by telephone from Aleppo.

"The rebels were not behind this attack," he said.

While the Russian Foreign Ministry supported the regime's version of events, the United States said on Tuesday that it had seen "no evidence" of Syrian rebels using chemical weapons.

White House spokesman Jay Carney said there was "no evidence to substantiate the charge that the opposition has used chemical weapons" and warned that it would be "totally unacceptable" for the regime to use such weapons, according to Agence France-Presse.

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The World Health Organization's Tarik Jasarevic told reporters on Tuesday that the agency "cannot confirm the use of chemical weapons, nor what agent, if any, was used" in Syria, but said medical supplies for victims of trauma were will be sent to the divided city of Aleppo on Wednesday, according to Reuters.

His comments came after CBS News quoted a Reuters photographer who had visited the University of Aleppo and the al-Rajaa hospital, where victims told him "people were suffocating in the streets and the air smelt strongly of chlorine."

The UK-based Syrian Observatory for Human Rights said on its Facebook page that it "heard through the wireless communication devices that 26 people (10 civilians, 16 regular soldiers) were killed when a rocket fell on the town of Khan al-Assal." The Observatory did not mention chemical weapons.

A member of the rebel Ansar brigade near in Khan al-Assal told Reuters via Skype what he had seen.

"We were about two kilometers from the blast. It was incredibly loud and so powerful that everything in the room started falling over. When I finally got up to look at the explosion, I saw smoke with a pinkish-purple color rising up."

Ahmed al-Ahmed also told the news agency that the missile, "maybe a Scud," hit a regime area, and that many rebel units denied having fired a chemical weapon.

If a chemical weapon attack is confirmed, it would be the first confirmed use of the agents in the two year conflict.

An unnamed UK Foreign Office spokeswoman commented on the reports of chemical weapons use:

"The UK is clear that the use or proliferation of chemical weapons would demand a serious response from the international community and force us to revisit our approach so far," the spokeswoman told Reuters.

The Syrian regime is suspected of having large stockpiles of chemical weapons.

The government is Russia's last Middle East ally. Although it denies supporting Damascus, Moscow has led efforts to block UN sanctions against Syria.