The global threat posed by the Islamic State group is still rising but US-based homegrown extremists pose the biggest danger to the homeland, Washington's top spy said Tuesday. 

In a report prepared for US lawmakers before he was due to address a Senate panel, Director of National Intelligence James Clapper said US-based extremists pose "the most significant Sunni terrorist threat."

These "homegrown violent extremists" or HVEs may be inspired by the attacks last year on military bases in Chattanooga, Tennessee and a workplace gathering in San Bernadino, California.

But they will also be influenced by the IS group's "highly sophisticated media" and by "individuals in the United States or abroad who receive direct guidance and specific direction from ISIL members or leaders."

A US-led military coalition is helping local forces in Iraq and Syria close in on cities in the IS group's so-called "caliphate," targeting its leadership and oil facilities with air strikes.

But, Clapper warned, the threat posed by the movement is only rising as it expands its reach into Libya and builds a global network of terror cells, supporters and allied armed groups.

"The Islamic State of Iraq and the Levant has become the preeminent terrorist threat because of its self-described caliphate in Syria and Iraq, its branches and emerging branches in other countries, and its increasing ability to direct and inspire attacks against a wide range of targets around the world," he said.

Clapper made the warning in the annual "World Wide Threat Assessment of the Intelligence Community," which he submitted to the Senate Armed Services Committee ahead of his appearance before it.