Gunmen have shot dead a Syrian general and two other senior officers in a day of violence that resulted in nearly 100 deaths across the country, the Syrian Observatory for Human Rights said on Thursday.

The general was killed on Wednesday on the road from Damascus to the Druze region of Suweida in the south, and a colonel was murdered in the central province of Hama.

The third officer, a major, was killed in the north near Aleppo, the country's second city.

"All three were in cars when they were targeted by the gunmen," Observatory chief Rami Abdel Rahman said.

At least 99 people, the majority of them civilians, were killed on Wednesday. The Britain-based Observatory said that among the dead were another two generals killed in combat.

It is not possible to independently verify death tolls for the conflict in Syria since the United Nations at the end of 2011 ceased compiling such figures.

However, the Observatory estimates that more than 16,500 people have been killed since the uprising erupted in mid-March last year.