U.S. Secretary of State John Kerry testifies before the Senate Foreign Relations Committee during a hearing on the authorization for the use of force in Syria on Capitol Hill in Washington D.C., capital of the United States, Sept. 3, 2013. U.S. President Barack Obama on Tuesday secured the key backing of House leaders in his push for military action in Syria, as his administration continued to persuade the rest of Congress by sending Secretary of State John Kerry and Secretary of Defense Chuck Hagel to make the case in a Senate hearing. (Xinhua/Zhang Jun)

U.S. Secretary of State John Kerry and Secretary of Defense Chuck Hagel on Tuesday made the case for a military strike against Syria, insisting inaction would undermine U.S. credibility, and advising lawmakers not to preclude option of putting boots on the ground under any circumstances.

"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.

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

"The United States must demonstrate through our actions that the use of chemical weapons is unacceptable," Hagel said, adding that refusing to act would undermine the credibility of other U.S. security commitments, including President Barack Obama's commitment to prevent Iran from acquiring nuclear weapons.

"The word of the United States must mean something," said Hagel. "It is vital currency in foreign relations and international and allied commitments."

The secretaries were cautious as to what an authorization from Congress should entail, as they promised the administration has no intention of participating in Syria's civil war.

"President Obama is not asking America to go to war," Kerry said. "He is asking for authorization to degrade and deter Bashar al-Assad's capacity to use chemical weapons."

Kerry said he did not want the resolution on the use of force in Syria to remove the option of putting "boots on the ground," in case an emergency such as to prevent chemical weapons from falling into wrong hands, a scenario representing a clear danger to the United States and its allies.

"I don't want to take off the table an option that might or might not be available to the president of the United States to secure our country," he said in answer to a question.

"In the event Syria imploded, for instance, or in the event there was a threat of a chemical weapons cache falling into the hands of al-Nusra or someone else, and it was clearly in the interests of our allies and all of us -- the British, the French and others -- to prevent those weapons of mass destruction (from) falling into the hands of the worst elements," said Kerry.

Al-Nusra is reported to be an al-Qaida affiliated group that operates in Syria.

Despite the warning, Kerry tried to assure lawmakers that Obama has no intention of sending troops to fight Syria's civil war.

"Whatever prohibition clarifies it to Congress or the American people, there will be no boots on the ground with respect to the civil war," said Kerry, in response to Senator Bob Corker's question.

Echoing Kerry, Hagel said the military objectives in Syria would be "to hold the Assad regime accountable, degrade its ability to carry out these kinds of attacks and deter the regime from further use of chemical weapons."

Hagel said the Defense Department has developed military options to achieve these objectives and positioned U.S. assets throughout the region to successfully execute this mission.

"We believe we can achieve them with a military action that would be limited in duration and scope," said Hagel, noting U.S. forces will be ready to act whenever the president gives the order.

Hagel underscored that the military force would not be used to resolve the underlying conflict in Syria -- an issue he said must be settled through a political solution by the Syrian people themselves.

He noted that Kerry is leading international efforts to help the parties move toward a negotiated transition, and expressed a commitment to "doing more to assist the Syrian opposition."

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