The jihadist rebound came as the United States and Russia issued a surprise joint presidential statement saying there was "no military solution" to Syria's grinding six-year war.
The two countries have long backed opposite sides in Syria, but the Kremlin on Saturday said US President Donald Trump and Russian counterpart Vladimir Putin made progress during a brief meeting on the sidelines of a summit in Vietnam.
Trump and Putin also "confirmed their determination to defeat ISIS".
The jihadist group overran swathes of territory in Syria and Iraq in 2014, but its self-styled "caliphate" has since been whittled down to a pocket of land along the border between the two countries.
ISIS is putting up a fierce defence there, particularly for the vital Syrian border town of Albu Kamal, said the Syrian Observatory for Human Rights monitoring group.
Syrian regime forces and allied militia from Lebanon, Iraq and Iran overran Albu Kamal on Thursday but lost the town again just two days later after a string of ISIS counter-attacks and ambushes.
"ISIS fully recaptured Albu Kamal, and regime forces and allied militia are now between one to two kilometres from the city limits," Rami Abdel Rahman, head of the Britain-based Observatory, said on Saturday.
ISIS Squeezed In Iraq
Across the border, Iraqi forces seized on Saturday several villages from the jihadists in an offensive to capture the last ISIS-held section of their country.
The operation's commander, General Abdelamir Yarallah, said his forces captured Rumana and 10 other villages as they worked their way towards the Euphrates Valley town of Rawa, the last Iraqi town still held by ISIS.