NAIROBI, Kenya (AP) — South Sudan's military has released six international aid workers who were arrested in January and accused of smuggling arms, according to the international medical organization Doctors Without Borders.

"We are relieved that our colleagues, who were working hard to bring health care to people in need, have now been released and can return to their families," said Brice de le Vingne, director of operations for the organization said in a statement to AP.

The aid workers were arrested on Jan. 4 in the Yei area on suspicion of transferring weapons to the opposition, charges denied by the Switzerland-based organization, also known as Medecins Sans Frontieres.

However South Sudan's information minister insisted the charges against the aid workers are valid.

"They were caught red-handed carrying ammunition and arms," Michael Makuei told AP by phone on Monday. He added the aid workers left Yei without permission and an investigation is ongoing.

Two of the international workers were released on Jan. 27, and the four others were released on March 31, said the organization.

Humanitarian organizations say there are facing increasingly hostile conditions in South Sudan, even as they are assisting civilians affected by the current famine and other crises. Famine was declared in two counties in South Sudan in February and more than 1 million people are on the brink of starvation, according to the U.N.

In March alone there were three serious attacks against aid workers in South Sudan, including the deaths of seven aid staff on March 25 when they were ambushed and killed in government territory.

"Intentional attacks against humanitarian workers may constitute war crimes," U.N. humanitarian chief Stephen O'Brien said on March 27.

The South Sudan government is suspicious of the activities of international non-governmental organizations. In February, Minister of Cabinet Affairs Martin Lomuro told AP he believed "most of the (humanitarian) agencies are here to spy on the government."

The Yei area where the Doctors Without Borders workers were arrested has seen a rise in ethnically targeted killings, according to the U.N. Monitors of the country's peace agreement found government forces responsible for burning thousands of homes.

At least 79 aid workers have lost their lives during South Sudan's civil war, which started in December 2013 and has killed at least 50,000 people.