WASHINGTON - US lawmakers expressed deep frustration on Wednesday over the wave of violence in South Sudan, questioning whether it made sense for Washington to continue sending hundreds of millions of dollars in aid to the fledgling democracy.

Four weeks of fighting, often along ethnic lines, has been 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.

US Representative Ed Royce, chairman of the House Foreign Affairs Committee, called the latest fighting "infuriating," and blamed it largely on South Sudanese leaders' unwillingness to build an inclusive state.

"It appears that the greatest threat to South Sudan post-independence is South Sudan itself," the California Republican said at a hearing on the turmoil.

The Senate Foreign Relations Committee held a similar hearing last week, at which lawmakers from that chamber similarly said the country risks losing US aid if its government and rebels do not end a wave of violence.

Washington has spent billions of dollars - congressional aides estimated $600 million per year - to help build the fledgling nation, including allowing weapons sales to its government and providing security training for its armed forces.

Unlike many African countries, South Sudan enjoys the strong interest of a broad range of US lawmakers, who backed the push by largely Christian and African southern Sudan to split from Muslim- and Arab-dominated northern Sudan and form the world's youngest state three years ago.

HEARING FROM CONSTITUENTS

But both Republicans, who hold the majority of seats in the House, and Democrats, who hold a majority in the Senate, are now questioning that support.

"We're starting to hear it more and more in our districts, that we've put so much time, attention, money into this situation ... and the outcomes seem to be terrible," said Representative Juan Vargas, a California Democrat.

Republican Representative Ted Yoho of Florida said Washington would do better to emulate China, which does not donate to South Sudan as the United States does, but is nonetheless the country's largest trading partner with major stakes in its oil industry.

US officials said US President Barack Obama's administration is putting pressure on both sides as well as on South Sudan's neighbors to reach a peaceful solution to the conflict.

"Neither the United States nor the international community will accept the armed overthrow of the democratically elected government of South Sudan," said Linda Thomas-Greenfield, Assistant US Secretary of State for African Affairs.

She said the international community is looking at options, including sanctions against individuals deemed responsible for the crisis, to hold accountable anyone responsible for human rights violations or blocking efforts to achieve peace.

"They are on both sides, within the government as well as those anti-government forces," she said.

The fighting since December 15 has pitted President Salva Kiir's SPLA government forces against rebels loyal to former Vice President Riek Machar, bringing the oil-exporting country close to civil war.

At least 1,000 people have been killed, with some estimates as high as 10,000, and more than 200,000 have been displaced. Oil exports - key to South Sudan's economy - have plummeted, adding to regional instability.

On Wednesday, Ugandan President Yoweri Museveni admitted for the first time to helping Kiir fend off the rebellion. Museveni said Ugandan troops had this week helped defeat rebels outside Juba, and some had been killed in battle. Museveni also blamed Machar for turning political differences into a military confrontation.