President Joe Biden said Sunday that the sudden collapse of the Syrian government under Bashar Assad is a "fundamental act of justice" after decades of repression, but it was "a moment of risk and uncertainty" for the Middle East.

Biden spoke at the White House hours after rebel groups completed a takeover of the country following more than a dozen years of violent civil war and decades of leadership by Assad and his family. Biden said the United States was monitoring reports of the whereabouts of Assad, with Russian state media saying he had fled to Moscow and received asylum from his longtime ally.

Hundreds of Syrian refugees gathered at two border crossings in southern Turkey on Monday, eagerly anticipating their return home following the fall of President Bashar Assad's government.

Many arrived at the Cilvegozu and Oncupinar border gates at daybreak, draped in blankets and coats. Some camped by the barriers of the border crossing, warming themselves with makeshift fires or resting on the cold ground. The border crossings correspond to the Bab al-Hawa and Bab al-Salameh gates on the Syrian side of the border.

Prime Minister Mohammed Ghazi Jalali, who remained in his post after Assad and most of his top officials vanished over the weekend, has sought to project normalcy.

“We are working so that the transitional period is quick and smooth,” he told Sky News Arabia TV on Monday, saying the security situation had already improved from the day before, when joyful crowds gathered in public squares and celebratory gunfire rang out across the capital.

A Syrian opposition war monitor says a top aide to the brother of Syria’s ousted president was found dead in his office near the capital, Damascus.

The Britain-based Syrian Observatory for Human Rights said Maj. Gen. Ali Mahmoud, who was in charge of Maher Assad’s office, had led a 2018 military campaign in southern Syria.

The Turkish-backed forces, known as the Syrian National Army, launched an offensive against the Syrian Democratic Forces in the northern Syrian city of Manbij on on Dec. 6, days after similarly expelling the SDF fighters from the city of Tal Rifaat.

Turkish security officials said Monday that “control of Manbij has been secured,” without providing further detail. The officials provided the information on condition of anonymity, in line with Turkish regulations.

The Israeli military said Monday that a drone that appeared to have originated in Yemen slammed into a city in central Israel.

Video posted by Israeli news sites showed a large burst of smoke erupting from a high-rise apartment building.

Abu Mohammed al-Golani, the militant leader whose stunning insurgency toppled Syria's President Bashar Assad, has spent years working to remake his public image, renouncing longtime ties to al-Qaida and depicting himself as a champion of pluralism and tolerance. As he entered Damascus behind his victorious fighters Sunday, he even dropped his nom de guerre and referred to himself with his real name, Ahmad al-Sharaa.

The extent of that transformation from jihadi extremist to would-be state builder is now put to the test.

Half a century of rule by the Assad family in Syria crumbled with astonishing speed after insurgents burst out of a rebel-held enclave and converged on the capital, Damascus, taking city after city in a matter of days.

Opposition forces swept across the country and entered Damascus with little or no resistance as the Syrian army melted away. President Bashar Assad, Syria's ruler for 24 years — succeeding his father, Hafez Assad — fled the country. Russian state media reported that he was in Moscow.

Syrian refugees across Turkey jubilantly welcomed the downfall of Bashar Assad's government Sunday, with many embracing the chance to return to their homeland.

Large crowds waving Syrian and Turkish flags gathered in the main square of Kilis, a border city in southern Turkey.

Syrian President Bashar Assad fled the country on Sunday, bringing to a dramatic close his nearly 14-year struggle to hold onto control as his country fragmented in a brutal civil war that became a proxy battlefield for regional and international powers.

The exit of the 59-year-old Assad stood in stark contrast to his first months as Syria's unlikely president in 2000, when many hoped he would be a young reformer after three decades of his father's iron grip. At age 34, the Western-educated ophthalmologist appeared as a geeky tech-savvy fan of computers with a gentle demeanor.