The United States has imposed sanctions on five South Sudan officials it believes are behind the abductions and likely murders in 2017 of two human rights activists.

Opposition member, Aggrey Idri and human rights lawyer Dong Samuel, were kidnapped from Nairobi, Kenya, in January 2017 by members of South Sudan's security services and were illegally transported to South Sudan's capital, Juba, said the U.S. The two were held at the main National Security Service prison before being moved to another national security run detention center where they were reportedly killed on orders from South Sudan's government, alleged the U.S. statement.

In a statement on Tuesday the U.S. Treasury said South Sudan's government has repeatedly used "extrajudicial killings as a means to silence dissent, limit freedom of speech and the press, and enforce the political status quo."

A United Nations panel of experts issued a report in April which concluded it is "highly probable" that the two men were executed by Internal Security Bureau agents on 30 January 2017, on orders from the commander of the National Security Service training and detention facilities.

South Sudan has denied any knowledge of what happened to the two men and does not show any signs of holding anyone accountable for their kidnapping or possible killing, said the U.S. statement.

South Sudan's government wasn't immediately available for comment on the sanctions.

South Sudan's leadership continues to undermine the stability and security of the country at the "expense of the lives, dignity, and prosperity of the Sudanese people," said the U.S. statement.

South Sudan is slowly emerging from a five year civil year that killed almost 400,000 people. A power-sharing agreement signed more than a year ago has been marked by delays and sporadic outbreaks of fighting in parts of the country. In November warring parties postponed the formation of a coalition government for the second time due to outstanding issues including security arrangements.

As a result of the delay, the United States said it would reevaluate its relationship with the country. Last month it recalled its Ambassador to South Sudan to Washington for consultations.

Some international observers welcomed the sanctions.

"Individual sanctions such as these send the message that serious human rights violations won't be tolerated and are an important step toward accountability," said Jehanne Henry, associate Africa director for Human Rights Watch. But it's South Sudan's government that has the responsibility to investigate these crimes, identify those responsible, and ensure perpetrators are held to account, she said.

"The people who abducted and murdered Dong Samuel and Aggrey Idri believed they could operate with impunity. They were wrong," said J.R. Mailey, director of investigations at The Sentry, a watchdog group that investigates corruption. But this is just one step on the path to accountability, he said. The people who assisted in the men's illegal deportation from Kenya to be investigated and held to account, he said.