The Islamic State consolidated its gains May 21, overrunning Iraqi government defenses east of Ramadi and seizing the last border crossing between Iraq and Syria controlled by the Damascus government.
The Syrian Observatory for Human Rights said IS control of the Al-Tanaf crossing means Syrian government forces have lost control over the porous border.
In addition, the militants spread out through Palmyra, beheading 17 people accused of "working with the regime," and taking over the cherished archaeological site, though there were no immediate reports of them destroying antiquities.
The observatory said the al Qaeda offshoot now controls more than half of all Syrian territory. Taking Palmyra gives IS access to modern army installations and control of a key desert highway.
But the areas it holds are mostly sparsely inhabited. Syria's main cities, including the capital Damascus, are located on its western flank, along the border with Lebanon and on the coast.