Syrian army deserters killed at least 27 soldiers and members of the security forces during clashes in the southern province of Daraa on Thursday, a rights group said.
The fighting broke out at dawn at checkpoints in three separate locations, the Britain-based Syrian Observatory for Human Rights said in a statement sent to AFP in Nicosia.
On Wednesday, army defectors killed at least eight Syrian troops in an act of revenge after security forces shot dead five civilians, in the second such attack in as many days, the Observatory said.
In its latest statement, the rights group said that among 21 civilians killed on Wednesday was an Iraqi woman shot by snipers on the outskirts of Damascus.
The clashes came as Human Rights Watch said in a report released Thursday based on interviews with former soldiers who defected. that Syrian military commanders have ordered troops to indiscriminately shoot at unarmed protesters.
The defectors named 74 military and intelligence officers "who allegedly ordered, authorised, or condoned widespread killings, torture, and unlawful arrests," the rights group said in a statement.
Troops were ordered to stamp out the anti-government demonstrations "by all means necessary", including lethal force, HRW said.
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It said that about half of the defectors interviewed were given direct orders to fire on both protesters and bystanders.
"Defectors gave us names, ranks, and positions of those who gave the orders to shoot and kill," Anna Neistat, HRW's associate director for emergencies, said in a statement.
Syrian President Bashar al-Assad has blamed the bloodshed on armed rebels, and HRW said it had documented cases where anti-regime forces have attacked government soldiers.
But, the rights group said, most of the protests it documented were largely peaceful, adding that the Syrian military commanders responsible for abuses committed against demonstrators must be held accountable.
The New York-based rights group said its report, "By All Means Necessary," was based on interviews with more than 60 Syrian former soldiers.
As the toll mounted, UN chief Ban Ki-moon urged world powers to act "in the name of humanity" against the crackdown, and the US State Department's special coordinator on Middle East affairs, Frederic Hof, likened the Damascus regime to a "dead man walking."
The United Nations said this week estimated that more than 5,000 people have been killed in the Syrian government's crackdown on dissent, which enters its 10th month on Thursday.