Machar has led a two-year rebellion against forces loyal to his longtime rival, President Salva Kiir
The U.S. State Department on Wednesday strongly condemned comments by South Sudan's opposition leader calling for a return to war against the government, saying it was "inexcusable" and showed a lack of concern for the well-being of the people.
Riek Machar, the country's former first vice president, and his allies ordered their forces to reorganize for "armed resistance" to the South Sudan government after talks over the weekend in Khartoum.
State Department spokesman John Kirby said Washington had seen Machar's statements and "strongly condemned" them, saying the violence of the past few years had proven fighting would not resolve the country's underlying conflict.
"We find it inexcusable that he would continue to promote armed resistance," Kirby told reporters. "It indicates a lack of concern for the well-being of the South Sudanese people, many of whom continue to struggle just to survive and just as much want to see peace."
Machar led a two-year rebellion against forces loyal to his longtime rival, President Salva Kiir, before the two sides reached a peace deal in August 2015. Under the deal, Machar returned to the capital Juba and resumed his role as vice president.
Fierce fighting flared between government and opposition forces in July, prompting Machar to withdraw from the country with his supporters. Kiir sacked him from his government position and named former opposition negotiator Taban Deng Gai as first vice president.