Louay Amokdad of the Free Syrian Army said the groups, which make up more than half the rebel forces, posted videos, starting Sunday, announcing their cooperation, CNN reported. He said one point of agreement is that taking Damascus, the country's capital, is critical to overthrowing President Bashar Assad.
The official government news agency reported a judge was killed in Damascus Wednesday when a bomb exploded under his car, in the second attack on regime officials in two days.
Judge Abad Nadhwah died instantly when the explosive was triggered by a remote device, the state-run news agency SANA reported.
Mohammad Osama al-Laham, a brother of the speaker of the Syrian Parliament, was killed Tuesday when bullets were fired into his car as he drove to work.
In addition, four mortar attacks by the Free Syrian Army shook government strongholds Wednesday in the Mezzeh neighborhood of Damascus.
Susan Ahmad told The Guardian rebels attacked an airport and a security department in Mezzeh, and attempted but failed to attack the presidential palace.
No deaths or injuries were reported.
Mezzeh is the home of many diplomatic and high-ranking officials in Syria, and is the location of some ministries, she said.
British Prime Minister David Cameron said Wednesday he wants to talk with Syrian rebels to help aid the opposition to Assad, the Financial Times reported.
Speaking to Syrian refugees at a camp in Jordan, Cameron said talks have been planned between John Wilkes, Britain's special envoy to the Syrian opposition, and rebel leaders.