Jeff Woodke, an American aid worker who has been held hostage in Africa since 2016, has been freed Monday, National Security Adviser Jake Sullivan announced. 

Sullivan tweeted that he is "gratified & relieved to see the release of U.S. hostage Jeff Woodke after over 6 years in captivity. 

"The U.S. thanks Niger for its help in bringing him home to all who miss & love him," Sullivan said. "I thank so many across our government who’ve worked tirelessly toward securing his freedom." 

Woodke was kidnapped from his home in Abalak, Niger, in October 2016 by men who ambushed and killed his guards and forced him at gunpoint into their truck, where he was driven north toward Mali’s border. 

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A senior administration official who briefed reporters Monday on Woodke's release described the action as the culmination of years of efforts but declined to say what exactly led to him being freed from captivity or where he is now. 

The official also said no ransom was paid, and no concessions were made as part of securing the 61-year-old's release. 

A source close to Woodke told Fox News in 2018 there was some indication that he was straddling the border between Niger and Mali and was being kept alive as something of a "bargaining chip." 

In an interview with the New York Times, Woodke’s wife Els, of McKinleyville, California, said she was told Jeff is in Niamey, the capital of Niger. 

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"He is safe," she said, adding "I don’t yet know if he is healthy." 

In a statement released through a family spokesperson, Els Woodke also said she "praises God for answering the prayers of Christians everywhere who have prayed for this outcome," the Associated Press reported. 

The FBI previously has said Woodke has worked in Niger for more than 25 years. 

The identities of Woodke's captors have not been disclosed. 

While no group has publicly claimed responsibility for Woodke's kidnapping, an array of jihadist militants with Al Qaeda ties are known to operate in the region.

Just prior to the announcement of Woodke's release, U.S. Secretary of State Antony Blinken visited Niger last week.

"It was wonderful to be in Niger this week," Blinken had tweeted Friday. "At a time of great challenge, this nation is an extraordinary model of resilience, democracy, and cooperation that we deeply value and respect."

Hollie McKay and the Associated Press contributed to this report.