(Adds Kerry comments)
JERUSALEM, Jan 5 (Reuters) - U.S. Secretary of State John Kerry voiced support for direct South Sudanese peace talks set to begin on Sunday and cautioned against any use of force to try to gain the upper hand.
The government of South Sudan and representatives of rebel forces met on Saturday evening in Ethiopia for the formal inauguration of peace talks, part of the diplomatic effort to halt weeks of fighting in the young nation.
More substantive, face-to-face bargaining is expected on Sunday.
"The negotiations have to be serious. They cannot be a delay gimmick in order to continue the fighting and try to find advantage on the ground at the expense of the people of South Sudan," Kerry told reporters during a visit to Israel.
"We will work to apply international pressure to any elements that attempt to use force to seize power. That is not acceptable," he said.
The fighting began on Dec. 15 between soldiers in a barracks in Juba. President Salva Kiir accused his former vice president, Riek Machar, of leading an attempted coup. Machar denied the charge, but his followers took up arms after the government detained a group of allied politicians.
In a statement on Saturday, the U.S. State Department called for "rapid, tangible progress on a cessation of hostilities" and urged the South Sudanese government to uphold its commitments and release political detainees immediately. (Reporting by Arshad Mohammed, Writing by Jeffrey Heller; Editing by Andrew Heavens)