(New throughout, adds details, background)
By Louis Charbonneau and Warren Strobel
UNITED NATIONS/WASHINGTON, Jan 10 (Reuters) - The United States is weighing targeted sanctions against South Sudan due to the failure of leaders in the world's youngest nation to take steps to end a crisis that has brought the country to the brink of civil war, sources briefed on U.S. discussions told Reuters.
"It's a tool that has been discussed," a source told Reuters on condition of anonymity about the possibility of U.S. sanctions against those blocking peace efforts or fueling violence in South Sudan. Another source confirmed the remarks, though both declined to provide details on the precise measures under consideration.
No decisions have been made yet, the sources added. Targeted sanctions focus on specific individuals, entities or sectors of country.
The U.S. government was unlikely to consider steps intended to economically harm impoverished South Sudan but would likely focus on any measures on those individuals or groups it sees as blocking efforts at brokering peace or committing atrocities.
Traditionally U.S. sanctions against individuals or groups involve a ban on travel to the United States and freezing of their assets in U.S. banks.
Three weeks of fighting, often along ethnic lines, is ringing alarm bells in Washington over the prospect that the conflict could spiral into full-blown civil war, spawning atrocities or making South Sudan the world's next failed state.
The fact that Washington is thinking of threatening U.S. sanctions against a country the United States helped create and supports with large amounts of aid shows how frustrated President Barack Obama's administration has become with President Salva Kiir and a rebel faction led by former Vice President Riek Machar.
Largely Christian South Sudan gained independence from predominantly Muslim Sudan in 2011 after a referendum was held in keeping with a 2005 U.S.-backed peace deal that ended a north-south civil war that left millions dead.
In a statement on Thursday the White House urged both sides in the escalating conflict to sign an agreement to cease hostilities immediately.
On Wednesday, South Sudanese rebels rejected a government plan to end a dispute over detainees and unblock peace talks. Fighting in the oil-producing nation has killed at least 1,000 people and caused hundreds of thousands to flee their homes, according to the United Nations.
South Sudanese President Salva Kiir, in turn, has refused to release 11 detainees despite promising Washington last month that he would free most of them.
U.S. President Barack Obama's National Security Adviser Susan Rice said in the White House statement that the United States was disappointed the detainees have not been freed yet but noted that their continued detention was no excuse to continue fighting.
"The United States reiterates its call upon President Salva Kiir to release the detainees immediately to the custody of (the East African trading bloc) IGAD so that they can participate in the political negotiations," Rice said.
U.S. government officials and senators said on Thursday that hundreds of millions of dollars in support to South Sudan's government could be stopped if the violence continues.
In 2012 the United States joined the other 14 members of the U.N. Security Council in threatening sanctions against Sudan and South Sudan due to a crisis over the disputed oil region Heglig, though council diplomats said Washington was reluctant at the time to back measures that would undermine Kiir's government.
U.S. officials said Washington was satisfied then that the mere threat of U.S. support for sanctions was sufficient. The Heglig crisis was eventually resolved. (Reporting by Louis Charbonneau and Warren Strobel; Editing by James Dalgleish and David Gregorio)