The White House said Tuesday there was no evidence of claims by the Syrian government that the oppositoin fighters had used chemical weapons, according to dpa. "We are deeply sceptical of a regime that has lost all credibility," spokesman Jay Carney said. "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." Carney said, however, that the United States was "looking carefully" at such claims, stressing that President Barack Obama remains concerned that al-Assad's regime could use chemical weapons as it becomes more beleaguered. He was reacting to Syrian government claims that 25 people had been killed in a chemical weapons attack by oppositoin fighters in the northern city of Aleppo. The state-run SANA news agency quoted Information Minister Omran al-Zoubi as alleging that "terrorists" had fired "a rocket containing chemical substances" on a government-held area in the city. French Foreign Ministry spokesman Philippe Lalliot said he had "no comment to make about a report coming from a Syrian state agency." The opposition Syrian Observatory for Human Rights said 26 people were killed in a rocket attack in Aleppo, adding that at least 16 were soldiers loyal to al-Assad. A spokesman for UN Secretary General Ban Ki-moon said he was aware of the report but could not confirm it. Western powers fear that al-Assad might use chemical weapons as he battles to stay in power.