The Syrian government did not comment on the new rebel command and throughout the uprising has considered the rebels terrorists backed by foreign powers that seek to destroy the country.

Assad’s regime appears increasingly embattled, with rebels making gains in northern Syria and near Damascus while the U.S., Turkey and others seek to hasten its demise. U.S. officials have expressed fears that Assad could use chemical or biological weapons, and U.S. intelligence has reported new activities at sites housing Syria’s chemical weapons.

The government tried to reverse the charge on Saturday, saying it had warned the United Nations that ‘‘terrorist groups’’ might use chemical weapons.

Syria’s state news agency said the Foreign Ministry had sent letters to the U.N. Security Council and U.N. chief Ban Ki-moon saying that Syria would not use chemical weapons against its own people.

Syria has never confirmed it has chemical weapons, but outside experts believe it has substantial stockpiles of mustard gas and a range of nerve agents, including sarin, a highly toxic substance that can suffocate its victims by paralyzing their lungs.

No rebel groups are known to possess chemical weapons and it is unlikely their mostly amateur fighters would know how to use them.