The United States announced on Tuesday it is imposing sanctions on two military leaders in South Sudan

The sanctioned leaders are Peter Gadet, a commander of rebel forces, and Marial Chanuong, commander of the government's Presidential Guard.

The two men are “responsible for perpetrating unthinkable violence against civilians,” Secretary of State John Kerry said.

He explained that Major-General Chanuong is being hit with financial sanctions for leading attacks on civilians in Juba.

South Sudanese soldiers massacred up to 300 Nuer men in Juba's Gudele district on December 19 of last year, Human Rights Watch has reported.

Secretary Kerry added that Gen Gadet, a defector from the South Sudan army, is being targeted for leading anti-government forces in the April 17 attack on Bentiu that killed more than 200 civilians. Most of the victims were reported to be Dinka.

The Obama administration's action, which bars US companies from doing business with Maj Gen Chanuong and Gen Gadet, serves to heighten pressure on Kenya and other East African countries to impose similar sanctions.

READ: EA’s headache over South Sudan crisis as US demands asset freeze

US officials who accompanied Secretary Kerry on his visit to South Sudan last week suggested that sanctions on South Sudanese figures would be more effective if also levied by Kenya and other neighbouring states.

Some South Sudanese responsible for fomenting violence are known to own property in Kenya, Uganda and Ethiopia, a US official noted last week.

And even if Kenya does not impose sanctions, financial transactions conducted in another African country by the targeted South Sudanese might involve a US institution and could thus be blocked, a US official noted.

The US also warned on Tuesday that sanctions against other prominent figures in South Sudan may be forthcoming.

A statement by UN Ambassador Samantha Power hinted that President Salva Kiir and rebel chief Riek Machar might be next.

"The measures taken against Marial Chanuong and Peter Gadet are only a first step and should serve as a clear warning to those in the government of South Sudan and those who have taken up arms against it: the United States is determined to hold accountable those who choose violence," Ambassador Power declared.

She then added: “President Salva Kiir and former Vice President Riek Machar have agreed to travel to Addis Ababa, Ethiopia, for face-to-face talks. We strongly urge both leaders to live up to this commitment to meet and to implement the Cessation of Hostilities agreement they signed on January 23.”