BEIRUT – Syrian army units have clashed with each other over following President Bashar Assad’s orders to crack down on protesters in Daraa, a besieged city at the heart of the uprising, witnesses and human rights groups said Thursday.

More than 450 people have been killed across Syria – about 100 in Daraa alone – and hundreds detained since the popular revolt against Assad began in mid-March, according to human rights groups.

While the troops’ infighting in Daraa does not indicate any decisive splits in the military, it is significant because Assad’s army has always been the regime’s fiercest defender.

It is the latest sign that cracks – however small – are developing in Assad’s base of support that would have been unimaginable just weeks ago. About 200 mostly low-level members of Syria’s ruling Baath Party have resigned over Assad’s brutal crackdown.

Ausama Monajed, a spokesman for a group of opposition figures in Syria and abroad, said the clashes among the soldiers have been happening since Monday.

“There are some battalions that refused to open fire on the people,” Monajed told The Associated Press, citing witnesses on the ground in Daraa, a city of 75,000 near the Jordanian border. “Battalions of the 5th Division were protecting people, and returned fire when they were subjected to attacks by the 4th Division.”

The 4th Division is run by the president’s brother, Maher.

The reports were corroborated by three witnesses in Daraa and an activist contacted by the AP. All four asked that their names not be used for fear of reprisals.

One of the witnesses said soldiers fired at each other Thursday around the Omari mosque in central Daraa. He said the soldiers from the 5th division, composed mostly of conscripts known to be sympathetic to residents, were battling soldiers of the 4th Division.

“They are defending the people against the forces of Maher Assad,” said the resident, who said he lived next to the mosque and witnessed the battles.