The U.S. government strongly believes Austin Tice, a Marine-turned-reporter who vanished in Syria in the summer of 2012, is alive and being held captive in the war-torn Middle Eastern country, according to a senior State Department official.

"I want to make it very clear that the United States government believes Austin Tice is alive," U.S. Special Presidential Envoy for Hostage Affairs Robert O'Brien told reporters on Tuesday. "We are deeply concerned about his well-being after six years of captivity."

In an event hosted by the National Press Club, O'Brien said the State Department is spearheading an investigative and diplomatic multinational effort to locate the missing journalist and secure his release. The special envoy called on the Russian government — which has provided significant financial and military assistance to the regime of Syrian President Bashar Al-Assad — to take part in this humanitarian cause.

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"There are plenty of areas of disagreements between the United States and Russia at this time," O'Brien said. "One of the things that both Russia and the United States should agree on is that innocent Americans, or innocent Russians for that matter, should not be held hostage and should not be held against their will."

Since 2011, Syria has been embroiled in a bloody and convoluted war involving forces loyal to President Al-Assad, moderate rebel groups, Iranian-backed Hezbollah cells, ISIS and Russian and American military units.

Tice, a former Marine Corps captain who served in Iraq and Afghanistan, traveled to Syria in the spring of 2012 to document the nation's then-young civil war as a freelance reporter before starting his final year at Georgetown Law School. After filing award-winning reports for various outlets — including McClatchy, The Washington Post and CBS News — Tice disappeared in Aug. 14, 2012.

Five weeks after his disappearance, a video surfaced showing an alive and blindfolded Tice surrounded by a group of unidentified armed militants.

In April of this year, when the FBI announced a $1 million reward for information "leading directly to the safe location, recovery, and return" of Tice, CBS News reported that although some believe he was captured by Syrian regime forces or pro-government militias, the circumstances surrounding Tice's disappearance remained a mystery.

Although he said he could not disclose intelligence information that supported the government's assessment that Tice was alive, O'Brien, the State Department envoy, highlighted the fitness of the combat veteran as a beneficial characteristic during this type of ordeal.

"He's got the toughness of a Marine. And I'm sure that's sustaining him through these incredibly trying circumstances," he said.

O'Brien noted that Secretary of State Mike Pompeo has been "intimately" and "actively" involved in the case and added that, along with U.S. Ambassador to the United Nations Nikki Haley and National Security Adviser John Bolton, America's chief diplomat has met with Tice's parents, Marc and Deborah Tice, on several occasions to brief the couple on any new developments.

Before the end of the year, Marc and Deborah Tice will undertake their seventh trip to the Middle East to apply for a visa to enter Syria. There, the couple hopes to be closer to Tice and reach out to whomever may be holding him captive.

"We continue our relentless effort to find the key that will open the door for Austin's freedom," Mr. Tice said.