BAGHDAD — Extremist fighters from the Islamic State in Iraq and Syria seized a military base in northern Syria on Sunday from forces loyal to President Bashar al-Assad, further solidifying control inside their self-declared Islamic state spanning the Syria-Iraq border.

The fall of the Tabqa air base followed the group’s seizing of two other Syrian military bases and gave it effective control of Raqqa Province, which abuts the Turkish border and whose capital city, Raqqa, has long served as the group’s de facto headquarters.

Recent military advances by ISIS in northern and eastern Syria have highlighted the lack of local military forces that can effectively battle the group, which President Obama last week called a “cancer” that must be eradicated from the Middle East.