The latest effort at peace in South Sudan is in danger of failure

The United States said it deeply regrets South Sudan's failure to sign a peace proposal by a Monday deadline, urged it to sign within 15 days and said Washington will look at how to raise pressure on those opposing the pact, Reuters reports.

The failure to reach a deal means that there's still no end in sight for what's turned into one of the world's most urgent humanitarian and political crises.

South Sudan became independent from the Republic of Sudan in July of 2011, as the result of a peace treaty that appeared to put an end to nearly five decades of conflict between the Khartoum government and the southern third of Sudan. But South Sudan has been in a state of civil war since December of 2013, when tensions between rival ethnic cadres within the ruling Sudanese People's Liberation Movement escalated into open conflict, and resulted in former vice president-turned rebel leader Riek Machar's defection from the government.

There's no reliable death count for the conflict since then - although the numbers of displaced have already reached the level of abstraction, with over 600,00 South Sudanese fleeing to neighboring countries on top of another 1.6 million made homeless within the borders of the world's newest independent state.

A peace agreement would determine how and whether Machar's forces can be reintegrated into the government. It entail reform for the SPLM and president Salva Kiir, who overrode constitutional term limits and began a new, unelected term as president in July.

Even in the event of a peace agreement though, there are still plenty of militants who aren't under the direct control of either Machar and the government - on top of numerous instances of fighting in the country that aren't directly related to the civil war. And ceasefires have been broken earlier in the conflict after being cautiously hailed as breakthroughs.

Even so, a peace agreement would at least demonstrate that some fitful progress is possible, and the US doesn't consider the negotiations to be a totally pointless exercise.

"The United States deeply regrets that the government of South Sudan chose not to sign ... We call on the government to sign the agreement within the 15-day period it requested for consultations," State Department spokesman John Kirby told reporters at his daily briefing, according to Reuters.

"We are going to work with our regional and international partners on the next steps and on ways to increase pressure, especially against those that are undermining the peace process or opposing this agreement."South Sudan President Salva Kiir declined to sign a peace deal proposed by regional leaders on Monday, saying he required more time.