Gen. Jameh Jameh, one of Syria's most powerful generals, died "carrying out his duties of national defense of Syria and its people" in the "prosecution of terrorists," the official Syrian Arab News Agency said.
The regime of Bashar Assad uses "terrorists" to describe rebels trying to overthrow the president, whose term ends in May 2014.
Lt. Col. Mohammad Abboud, a top rebel commander, was quoted by The Washington Post as saying Jameh, 59, was fatally shot near his home in the al-Joura neighborhood of Deir Ezzor, Syria's sixth-largest city and the capital of the Deir Ezzor province, which borders Iraq.
Deir Ezzor, on the shores of Euphrates River, is 280 miles northeast of Damascus, Syria's capital.
The opposition Syrian Observatory for Human Rights, based in Britain, said rebel snipers shot Jameh in a battle.
The European Union imposed sanctions on Jameh two years ago, saying he was "directly involved in violence against the civilian population" of Deir Ezzor.
His death came as Syrian Deputy Prime Minister Qadri Jamil was reported to have said in Moscow long-postponed peace talks in Switzerland under international auspices -- known as Geneva II -- could be held Nov. 23-24.
The United Nations and the United States said reports by Russian news agency RIA Novosti and SANA were premature.
"When it is time for an announcement, the secretary-general will make one," said Martin Nesirky, a spokesman for U.N. Secretary-General Ban Ki-moon spokesman.
Ban is expected to lead the conference.
"No date is final until it is set and announced by the U.N.," State Department spokeswoman Jen Psaki told reporters at a briefing Thursday. "We've said for a couple of weeks now that we were targeting mid-November, and that remains the case."
Diplomacy surrounding Syria has gathered pace since September, when Moscow and Washington worked out a deal for Damascus to give up its chemical weapons.