US Secretary of State John Kerry presents the administration's case for US military action against Syria to a Senate Foreign Relations Committee hearing in Washington September 3, 2013.[Photo/Agencies]

WASHINGTON - US Secretary of State John Kerry said on Tuesday a military strike against Syria for alleged chemical weapons use is not about President Barack Obama's "red line," but because the use of chemical weapons cannot be tolerated.

"This is not the time for armchair isolationism ... We have spoken up against unspeakable horror. Now we must stand up and act, " Kerry told a Senate Foreign Relations Committee hearing on Syria,

Secretary of Defense Chuck Hagel, who testified after Kerry, said the alleged use of chemical weapons in Syria represents " grave risks" and "serious threats" to the US national security interests and its allies'.

The military, Hagel said, has positioned assets in regions close to Syria and is confident of achieving objectives set for a strike in Syria.

He admitted there is risk in carrying out the military action, but said inaction would undermine credibility of other US commitments.

"The word of the United States must mean something," Hagel said.

Protesters who opposed the military strike have interrupted Kerry's remarks at the hearing.