The United States vowed on Tuesday to "work tirelessly" to bring home missing American journalist Austin Tice, who disappeared in Syria in 2012, and appealed to his captors to free him.

In a statement marking Tice's 1,000th day in captivity, White House National Security Council spokeswoman Bernadette Meehan called on governments and individuals with information about his whereabouts to "work cooperatively with us to help bring him home."

Meehan said U.S. officials "strongly urge Austin's captors to release him."

"The United States government will continue to work tirelessly to bring Austin home to his parents, Debra and Marc, and his brothers and sisters, who have endured anguish and suffering since Austin's abduction," Meehan said.

The White House said Tice, a U.S. Marine Corps veteran, was abducted in August 2012 while reporting from a suburb of Damascus during the Syrian civil war. He has not been heard from other than a brief video that surfaced five weeks later. It is unclear who is holding Tice. His family has said they believe he is still alive.

In March, a senior State Department official said the United States had been in direct, periodic contact with Syrian government officials to discuss Tice.

The official declined to say how many more U.S. citizens were missing or taken hostage in Syria, but said U.S. officials were working with Czech diplomats, serving as intermediaries between the United States and Syria, on their whereabouts and welfare.

"We greatly appreciate the efforts of the Czech government, which acts as the U.S. protecting power in Syria, on behalf of our citizens, including Austin," Meehan said.

Tice is one of several American and other foreign journalists and aid workers to go missing in Syria during the civil war. The Islamic State militant group has executed some of them.