LONDON — U.S. Defense Secretary Lloyd Austin said Monday that the international community was “surprised to see that the opposition forces moved as quickly as they did” in Syria as President Bashar al-Assad’s government collapsed in the face of a surprise rebel offensive.

“Everybody expected to see a much more stiff resistance from Assad’s forces,” Austin said while in Japan, during what is expected to be his last trip to the Indo-Pacific region as defense secretary.

The speed of developments, he added, “was surprising, I think, to most everybody in the international community.”

Damascus’ fall to rebel forces on Sunday marked “a moment of historic opportunity for the long-suffering people of Syria to build a better future for their country,” President Joe Biden said in a post to X, adding, “It is also a moment of risk and uncertainty.”

U.S. forces are already moving to suppress any hint of an ISIS resurgence in central and eastern Syria, where hundreds of American personnel have been active for several years alongside Kurdish forces to defeat the remnants of the jihadist group.

U.S. forces launched 75 strikes on ISIS targets in central Syria on Sunday to “disrupt, degrade and defeat” the group, the head of the U.S. Central Command said in a statement.

Austin said the strikes were designed “to keep the pressure on ISIS.”

“As this unfolds, there’s a potential that elements in the area, such as ISIS, could try to take advantage of this opportunity and regain capability,” he explained.

“We’ve been tracking ISIS as a part of our ‘Defeat ISIS’ campaign for some time, as you know, and we’ve seen cells trying to strengthen and develop additional capability out in the Vidalia Desert and those strikes were focused on those cells,” Austin said.

U.S. forces are “still evaluating the results, but I think that we’re going to find that we’ve been pretty successful,” Austin said.