About 3,000 civilians were evacuated Thursday on buses and ambulances from eastern Aleppo, officials and human rights groups said, marking the first step for rebels to surrender the decimated enclave in Syria's second-largest city.

As the caravans rolled out, Secretary of State John Kerry said it appeared a key cease-fire deal allowing for people to be transported out of eastern Aleppo was holding.

He told reporters Thursday afternoon that a first wave of 21 buses and 19 ambulances reached a checkpoint via a safe passage route. That group includes more than 1,000 people on their way to the Turkish border.

Kerry said the regime of Syrian President Bashar Assad is committing "nothing short of a slaughter" as the nearly six-year-long civil war rages on. But he reiterated that the fall of Aleppo wouldn't mean the nation is secure.

"How many countries will step up and rebuild it to the policies that are being executed today?" he asked, adding that the U.S. is open to a cease-fire across all of Syria.

SAMS medical staff in #Hama are receiving the evacuated civilians from #Aleppo & transporting them to our facilities #StandWithAleppo pic.twitter.com/V7lZsZONYh — SAMS (@sams_usa) December 15, 2016

In a new video posted on the Syrian presidency's Telegram channel, Assad painted the evacuation as a success, and said "the history that is being made now is bigger than the word congratulations."

The last remaining rebels in eastern Aleppo, along with tens of thousands of civilians, are being evacuated under a cease-fire deal reached with Russia, a close Assad ally. The country has been ravaged by bloody fighting, but rebels finally ceding control in eastern Aleppo would be a major turning point in the battle.

Of the 3,000 people, including children, evacuated from eastern Aleppo as of Thursday, 40 were wounded, according to the International Committee of the Red Cross (ICRC).

Vehicles were returning back and forth Thursday to evacuate more people, the agency said.

"When we arrived, the scene was heart-breaking. People are faced with impossible choices," the ICRC's Marianne Gasser, the head of the Syria delegation, said in a statement. "You see their eyes filled with sadness. It was very moving."

The Britain-based Syrian Observatory for Human Rights monitoring group said vehicles crossed from government-held areas shortly after noon local time (5 a.m. ET).

Related: Syrians Trapped in Aleppo Plead for Help Out of 'Disaster'

Guns fell almost silent in the devastated city overnight, part of an agreed cease-fire to allow for the evacuation. But the humanitarian group White Helmets said one of its volunteers had been shot by a government sniper while clearing an ambulance route.

The White Helmets humanitarian group said a volunteer had been shot and injured by a Syrian government sniper while clearing an ambulance route.

An ambulance official said a convoy was fired at as it prepared to leave eastern Aleppo, wounding three people, but it was not clear whether he was referring to the same incident.

"The convoy was shot at by regime forces and we have three injured ... They were brought back to besieged areas," Ahmed Sweid told the pro-rebel Orient TV.

A Reuters witness in nearby government-held territory heard a burst of gunfire that lasted several minutes.

Russia’s Defense Ministry broadcast webcam images of what it said was an evacuation corridor in the ar-Ramuseh area of the city showing waiting ambulances.

It said authorities had guaranteed the safety of rebels and their families who will be evacuated toward Idlib, a city in northwestern Syria.

The evacuations came after an earlier cease-fire deal, mediated by Turkey and Russia, unraveled Wednesday. Plans were abandoned after shelling and airstrikes were followed by retaliation from rebels.

Residents, activists and medical staff described mayhem at the time as volleys of shells rained down on the area where tens of thousands of civilians were trapped.