HOUSTON — The parents of detained freelance journalist Austin Tice tell his captors in Syria that American authorities will feel "sincere appreciation" if he returns home.

It’s been more than six years since Tice, who served in the U.S. Marine Corps, went missing near Damascus, Syria, in August of 2012. His parents, Marc and Debra Tice, continue to fight for his release, staying in contact with the Syrian government and making frequent trips to the Middle East.

“It's important for those that are in authority over Austin's release to know that no one is benefiting from his detention,” Debra said. “But when he is released, there will be great benefit and sincere appreciation from the United States government.” She notes that, “there’s no doubt in anyone’s mind” that her son is alive.

A State Department official told NBC News, “We are still operating under the assumption Austin is alive, and are still liaising through our Czech colleagues to press the Assad regime to acknowledge his detainment and release.”

Freelance journalist Austin Tice went missing in Syria in 2012 and has not been heard from since. Fort Worth Star-Telegram / TNS via Getty Images

His parents said that they have spoken to President Donald Trump and Vice President Mike Pence personally, who have told them they are committed to doing all they can to bring Tice home.

NBC News first spoke with Tice’s parents in the summer of 2017, when they told Lester Holt they hoped to give their son his birthday cards in person that year. “Every day is a good day for Austin to come home,” Marc Tice said then.

The Tices continue to raise awareness of his case, most recently working with the National Press Club, to host “Night Out for Austin Tice.” On Thursday, the eve of World Press Freedom Day, 79 restaurants in 13 states and the District of Columbia donated a portion of their funds, in hopes of doubling the FBI reward for information leading to Tice’s safe return.

The FBI is currently offering a reward of up to $1 million.

Since the FBI issued this reward in April of 2018, the Tices say it has led to information to the FBI and the family.

“The fact of the matter is, we know that the reward has generated information, useful information for Austin,” Marc Tice said. “We know that increasing that reward should increase and we hope that it will increase the information that's shared, and that information will speed Austin's return and lead to his safe return.”

Chris Shepherd is the owner of four restaurants in Austin’s hometown of Houston. He contributed 10 percent of his sales from his restaurants Thursday evening. “It's time to bring awareness back to it, and get him back to being back home here in Houston,” Shepherd said.

Marc and Debra Tice acknowledge that it is difficult to maintain awareness of their son’s case in public, but they hoped the Night Out campaign will motivate folks to encourage the White House, the State Department and Congress to press for his release.

“They may have heard about it six years ago, or five years ago, or four years ago, but they don't realize that Austin is still waiting to walk free. He's staying alive, he's holding onto hope because he's looking forward to being free again,” Debra Tice said.

“It’s gonna take reminding the people in our government especially, keep at it. Keep at it until we see him back in his mom's arms."

Cornett reported from Houston, Cho from New York.