Israeli security sources said on Tuesday that the reported use of chemical weapons in an attack near Syria's Aleppo looks reliable, but stopped short of confirming the incident. Also on Tuesday, Russia directly accused Syrian rebels of using chemical weapons in the morning attack, and said it was an extremely alarming and dangerous development.

The Syrian government and rebels accused each other of launching a deadly chemical attack near the northern city of Aleppo, in which at least 25 people were killed early on Tuesday. 

The United States said on it was evaluating allegations of chemical weapons use in the attack but dismissed charges that the opposition had used such weapons in the two-year-old conflict.

"We are looking carefully at allegations of ... chemical weapons use, we are evaluating them," White House spokesman Jay Carney told reporters.

"We have no evidence to substantiate the charge that the opposition has used chemical weapons," he said.

"We are deeply skeptical of a regime that has lost all credibility and we would also warn the regime against making these kinds of charges as any kind of pretext or cover for its use of chemical weapons."

The State Department echoed those comments and the Pentagon said it was monitoring the situation.

"I have no information at this time to corroborate any claims that chemical weapons have been used in Syria," Pentagon spokesman George Little said. "The use of chemical weapons in Syria would be deplorable."

Carney reiterated that President Barack Obama has said there  would be consequences and the government of President Bashar al-Assad would be held accountable if chemical weapons were used. Carney would not say what those consequences would entail.The United States has been concerned that the Assad government would consider using chemical weapons as it becomes  "increasingly beleaguered and finds its escalation of violence through conventional means inadequate," Carney said. "This is a serious concern."

He said the U.S. position is still that it is supplying only non-lethal aid to the Syrian opposition. "Our position is and remains that we are not supplying lethal assistance to the opposition," Carney said.

Earlier Tuesday, Russia accused Syrian rebels of using chemical weapons in the morning attack, and said it was an extremely alarming and dangerous development. 

"A case of the use of chemical weapons by the armed opposition was recorded early in the morning of March 19 in Aleppo province," the Russian Foreign Ministry said after President Bashar Assad's government and rebels accused each other of launching a deadly chemical attack.

"We are very seriously concerned by the fact that weapons of mass destruction are falling into the hands of the rebels, which further worsens the situation in Syria and elevates the confrontation in the country to a new level," the ministry said in a statement.

Russia has been the main ally of President Bashar Assad's regime since the start of the uprising, in which more than 70,000 have died. 

Moscow has warned Assad's government not to use chemical weapons and said in December that Damascus had taken steps to ensure that chemical agents were secure by concentrating them at a smaller number of sites. 

Syria's state news agency accused rebels earlier Tuesday of using chemical weapons in the attack. 

"Terrorists fired a rocket containing chemical substances in the Khan al-Assal area of rural Aleppo and initial reports indicate that around 15 people were killed, most of them civilians," SANA news agency said in an initial report.

A Syrian rebel commander denied reports that the opposition forces were behind the chemical weapon attack in Aleppo, saying the government had fired a rocket with chemical agents on the town of Khan al-Assal.

"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. Then suddenly we learned that the regime was turning these reports against us," said Qassim Saadeddine, a senior rebel and spokesman for the Higher Military Council in Aleppo. "The rebels were not behind this attack."

Meanwhile, Britain's UN envoy said that the reports of a chemical weapon attack had not yet been "fully verified."

"We have seen those reports, they haven't yet been fully verified," Britain's UN Ambassador Mark Lyall Grant told reporters on his way into a UN Security Council meeting on Afghanistan.

"But clearly if chemical weapons were used then that would be abhorrent and it would require a serious response from the international community," he said.

If confirmed it would be the first use of such weapons in the two-year-old conflict.