Defense Secretary Chuck Hagel offered condolences to the family of the Army Ranger-turned-aid worker who was beheaded by Islamic extremists and said the tragedy should renew the nation's focus on defeating the Islamic State.

"He was an Army Ranger and his entire life was about helping others," Hagel told reporters on a flight to the Army's Fort Campbell Sunday night.

A video released online Sunday showed a masked militant standing over the severed head of a man and saying it was former Private First Class Peter Kassig.

Kassig enlisted in the Army in 2006 and served with the 75th Ranger Regiment, a special operations unit. He deployed to Iraq in 2007 and was medically discharged as a private first class later that year.

In 2012, Kassig returned to the Middle East and founded an aid group to help Syrians caught in their country's brutal civil war. A certified EMT, Kassig delivered food and medical supplies and provided trauma care to wounded Syrians before being captured in eastern Syria in October 2013.

Hagel applauded Kassig's commitment to service.

"I think the fact that he went back to [to the Middle East] to help the people that he saw that needed help says everything about who he was and what he believed he believed," Hagel said.

"There are people with extraordinary values and principles in their lives, like Peter Kassig, and he applied that and it cost him his life," Hagel said.

"I know how much he'll be missed by his family, and his family will always be a part of the Army Ranger family," Hagel said.

In previous videos showing the beheadings of the two American journalists and two British aid workers, the hostages were shown kneeling in orange jumpsuits as they were forced to make speeches before their killer lifted a knife to their throats.

The latest video did not show Kassig being beheaded. And unlike previous videos, it did not show other Western captives or directly threaten to behead anyone else. It also had lingering close-ups on some militants' exposed faces, a few of whom appeared to be foreigners.

The video appeared on websites used in the past by the Islamic State group, which now controls a third of Syria and Iraq.

The Islamic State group still holds other captives, including British photojournalist John Cantlie, who has appeared in several videos delivering statements for the group, likely under duress, and a 26-year-old American woman captured last year in Syria while working for aid groups. U.S. officials have asked that the woman not be identified out of fears for her safety.

The video appeared to be part of continuous efforts to strike at the U.S., which is leading an aerial campaign against the group that began in August in Iraq and spread to Syria the following month.

The video came two days after a recording by the group's leader, Abu Bakr al-Baghdadi, was uploaded to the Internet. The militant leader warned that the U.S.-led coalition's campaign had failed and it would eventually have to send ground troops into battle.

The group has declared a self-styled Islamic caliphate in areas under its control, which it governs according to its violent interpretation of Shariah law, including massacring rebellious tribes and selling women and children of religious minorities into slavery.

The group's militants have also beheaded and shot dead hundreds of captives, mostly Syrian and Iraqi soldiers, during its sweep across the two countries, and has celebrated its mass killings in extremely graphic videos.

The Islamic State group has its roots in al-Qaida's Iraqi affiliate but was expelled from the global terror network over its brutal tactics and refusal to obey orders to confine its activities to Iraq. It became even more extreme amid the bloody civil war in neighboring Syria and grew strong enough to launch a lightning offensive across Iraq.

Syria's war began as an uprising against President Bashar Assad. Activists say that conflict has killed more than 200,000 people.

The Associated Press contributed to this report.