The head of Syria’s military police said in video footage aired by the al-Arabiya network late Tuesday that he has defected and joined the rebel forces battling the government of President Bashar al-Assad.

Maj. Gen. Abdul-Aziz Jassem al-Shallal is one of the highest-ranking officers to leave the Syrian military, and his defection appeared to signal growing disillusionment among senior commanders about the outcome of the conflict that has left at least 40,000 Syrians dead.

In recent weeks, the Syrian military has adopted increasingly heavy-handed tactics, including firing Scud missiles at rebel positions — a development that observers say reflects the government’s desperation. Rebel forces have also made notable gains, taking control of more than half a dozen military bases and expanding their arsenal with confiscated weapons.

Shallal’s defection comes just weeks after 13 Syrian military officers crossed into Turkey on Dec. 7. The spokesman for the Syrian Foreign Ministry, Jihad Makdissi, also left the country early this month under circumstances that the government characterized as a leave of absence but that opposition activists called a defection.

In the video, Shallal is wearing his military uniform and appears calm, saying he had left the military because of “the diversion of the army from its basic mission of protecting the country to become gangs of murder and destruction.”

In a phone interview with the al-Arabiya satellite network, Shallal said that numerous senior government officials would like to defect but are not able to because of close monitoring by the Syrian security forces.

“The situation is not allowing them to,” he said.

In Washington, acting State Department spokesman Patrick Ventrell said that the United States has been unable to confirm Shallal’s actions or whereabouts but added that the reports of his defection, if true, “would be yet another sign of the regime crumbling from within, as those around Assad realize that the end of his rule is inevitable.”

Getting Shallal out of Syria into Turkey was a complicated and dangerous operation that was planned and coordinated by the Free Syrian Army, a rebel official said. Loay Mokdad, a spokesman for the rebel force, said the trip took four days and involved the use of motorcycles and horses, as well as a long trek through the mountains between the two countries.

“We welcome all defections and are ready to help, because there is no reason to be afraid of regime retaliation anymore,” Mokdad said, adding that the regime will “end in a few weeks.”

Shallal’s defection coincided with attacks across the country by the Syrian military that left at least 100 people dead, according to the Local Coordination Committees activist network.

At least 20 people were killed Wednesday in the village of Qahtaniya, in northern Syria, when the military hammered the area with artillery shells, opposition groups said. A gruesome video posted online showed several bloody bodies, including those of children, laid out on a floor.

Timeline: Major events in Syria’s tumultuous uprising that began in March 2011.

Despite the apparent unwillingness of both sides to lay down their arms, U.N. envoy Lakhdar Brahimi arrived in Damascus on Sunday in the latest effort to negotiate a political solution to the conflict. Brahimi met with several opposition figures in Damascus on Tuesday and is scheduled to travel to Russia, one of the Syrian government’s staunchest backers, on Saturday.

Ahmed Ramadan and Suzan Haidamous in Beirut and Joby Warrick in Washington contributed to this report.