More than 10,000 soldiers have deserted the Syrian army, and as many as half of those conscripted did not report for duty in the last three call-ups, Western intelligence agencies told Israel's Haaretz daily.
High-ranking officers remained loyal to beleaguered President Bashar Assad, but lower-level officers have deserted in large numbers, the agencies said. In some cases, units have deserted en masse.
The White House released a statement Wednesday calling for the "full withdrawal of security forces, the release of political prisoners, and unfettered access by monitors and international media to all parts of Syria."
"The United States continues to believe that the only way to bring about the change that the Syrian people deserve is for Bashar al-Assad to leave power. The words of the Assad regime have no credibility when they continue to be followed by outrageous and deplorable actions," the White House said, adding Assad "should have no doubt that the world is watching, and neither the international community nor the Syrian people accept his legitimacy."
At least 60 Syrian soldiers attempting a mass desertion were mowed down with machine-gun fire by loyalist soldiers, opposition groups said Tuesday.
The Idlib province massacre -- one of the deadliest in the country's nine-month uprising -- occurred Monday as hundreds of soldiers tried to flee across the Turkish border to join the Free Syrian Army, a rebel force made up of fellow defectors, opposition forces said.
Those who escaped were hunted down Tuesday, Britain's Daily Telegraph reported.
Eighty defected soldiers were also killed Tuesday, Capt. Ahmed al-Hasan of the Free Syrian Army told The Washington Post.
At least 73 other people were killed in Syria Tuesday in clashes between the army and opposition, most of them in Idlib, in the northwest bordering Turkey, and Homs in the central west.
An activist with the rights group Avaaz told the Post local activists and medical groups put the death toll Tuesday at 269 -- 97 security forces, nine civilians and 163 defected soldiers who staged ambushes against loyalist forces.
Violence Monday left 100 people dead, including 14 soldiers ambushed by opposition forces, human-rights groups said.
Syria restricts journalist access, so it is difficult to verify or assess sometimes-conflicting accounts.
China's state-run Xinhua news said tens of thousands of Syrians were gathered in Damascus Wednesday to voice their support for Assad's government.
More than 5,000 people have been killed in the Syrian unrest, most of them anti-Assad activists, the United Nations estimates.
Assad, facing international condemnation for his brutal crackdown on opposition to his rule, claims the activists are foreign-backed "armed terrorists."
Rights groups said Assad's forces began transferring wounded opposition activists from hospitals to army bases to prevent them from testifying to Arab League observers, expected to arrive starting Wednesday.
The 150 observers -- who the league said would be in place after the weekend -- are being sent to ensure the army and opposition adhere to a cease-fire and end the violence.
Syria reluctantly agreed Monday to let in the foreign observers after pressure from close ally Russia and the Arab League, which threatened Saturday to refer Damascus to the U.N. Security Council. The council was waiting for word from the league before moving ahead with a sanction resolution on Syria.
In another development, the Assad regime announced a law recommending the death penalty for anyone found arming the anti-regime "terrorists," Syrian state TV reported Tuesday.
Besides the death penalty, the law says those involved in arms smuggling "for profit or to carry out acts of terrorism" face life imprisonment and hard labor, state TV reported.