Any use of chemical weapons in Syria is a “serious concern” and the U.S. is investigating claims by the Assad regime, White House press secretary Jay Carney said yesterday.

Syria’s Ambassador to the United Nations, Bashar Jaafari, accused “terrorist groups” of firing a rocket laden with chemicals, whose “thick smoke” killed 25 people and injured 110 in the Khan al-Assal area in Aleppo province. Jaafari made the allegation in a letter sent yesterday to UN Secretary- General Ban Ki-moon and this month’s president of the decision- making Security Council, Russian UN envoy Vitaly Churkin.

The Russian Foreign Ministry gave credence to the Syrian government’s version of events, saying in a statement that it was concerned “that weapons of mass destruction have fallen into the hands of fighters.” Russia is an arms supplier to Syria and has vetoed three resolutions at the Security Council seeking to threaten the regime with punitive sanctions.

The Obama administration, which has criticized Russia for supporting Assad, disagreed.

The U.S. doesn’t “have any information to suggest that the opposition has the capability to do what the regime has alleged,” Victoria Nuland, spokeswoman for the State Department, told reporters yesterday.

Obama’s ‘Red Line’

The two attacks were the work of the Assad regime and should be the subject of an international inquiry, including a visit to the sites, the opposition Syrian National Coalition said in an e-mailed statement.

“The interim government stresses its willingness to receive this delegation on Syrian territories, and ensure secure and safe passage,” it said.

Obama has told the Syrian government that its use of such weapons would cross a “red line.”

“The president was clear when he said that if Assad and those under his command make the mistake of using chemical weapons or fail to meet their obligations to secure them, then there will be consequences and they will be held accountable,” Carney said.

‘Serious Response’

The U.K., which along with France is shifting toward more active assistance to the rebels, underscored the warning against any use of such weapons in Syria.

“Clearly, if chemical weapons were used then that would be abhorrent and it would require a serious response from the international community,” Britain’s UN Ambassador Mark Lyall Grant told reporters yesterday in New York.

While Syria’s regime has produced, stored and weaponized chemical weapons, “little is known from open sources” about the size and condition of the stockpile, the nonpartisan U.S. Congressional Research Service said in a report in December.

The regime reportedly has stocks of nerve and blister agents such as sarin, VX and mustard gas.

The Syrian opposition -- which has just elected a new interim prime minister, Ghassan Hitto -- has accused Assad’s forces of using chemical agents over the course of the two-year conflict. There have been no confirmed cases, and it wasn’t determined whether such allegations referred to deadly nerve agents such as sarin gas or nonlethal irritants such as tear gas.