In a complaint unsealed this week and dated February 29, the prosecutors alleged that Peter Ajak, a former economist with the World Bank, and an associate, Abraham Chol Keech, intended to send the weapons to "opposition groups seeking to effect a non-democratic regime change in South Sudan."
Ajak, 40, has been a fierce critic of South Sudanese President Salva Kiir's government. He was jailed in 2018 and charged with treason.
Those charges were dropped but he was convicted of disturbing the peace over interviews he gave to foreign media. Kiir later pardoned him.
The Justice Department alleged in the complaint that the defendants violated U.S. law making it illegal to export weapons to South Sudan, which is subject to a U.N. arms embargo.
The country has endured years of civil war and fighting between armed groups since winning independence from Sudan in 2011.
It was not immediately clear if Ajak and Keech were represented by counsel.
A statement released by Revive South Sudan, an advocacy group led by Ajak said his arrest was “administrative detention” pending investigations and court proceedings.
Ajak will soon be released to defend himself in court, the statement added.
Between February 2023 and February 2024, Ajak and Keech tried to buy weapons from undercover law enforcement agents and smuggle them to South Sudan through a third country, according to the justice department complaint.
As part of the alleged scheme, they agreed to an arms contract worth nearly $4 million and requested a "fake contract" that said the funds were for things like equipment related to "human rights, humanitarian, and civil engagement inside South Sudan refugee camps," the complaint said.
Ajak fled to the United States during South Sudan's struggle for independence. He is an asylee who serves as a fellow at U.S. university and regularly authors articles regarding the East African nation’s political and economic future.
Keech is a naturalized U.S. citizen who currently residents in Utah. He serves as coordinator for opposition groups in South Sudan.
VOA reached Michael Makuei, South Sudan’s minister of information and government spokesperson, who failed to comment because he was in a meeting.
Some information for this report was sourced from Reuters. Reporter Denis Logonyi contributed to this report.