"The council has made it very clear: that it is prepared to impose consequences if there continue to be spoilers, if there continue to be people carrying out gross violations of human rights," US ambassador to the UN Samantha Power said after meeting with President Salva Kiir.
Representatives of the 15-member council, who arrived in the capital Juba today on a two-day mission, are due also to meet rebel chief Riek Machar.
"We have delivered that message here, we will deliver it to Riek Machar," she added.
Thousands of people have been killed and over 1.5 million have fled almost eight months of carnage sparked by a power struggle between Kiir and his sacked deputy Machar, with battles between government troops, mutinous soldiers and ragtag militia forces divided by tribe.
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"The parties must know that the people of South Sudan have suffered enough," Rwandan ambassador Eugene-Richard Gasana told reporters.
The ambassadors later left to visit the northern town of Malakal, one of the hardest hit areas in the fighting.
The town has been left in ruins after swapping hands several times between government and rebels.
Foreign Minister Barnaba Marial told reporters the diplomats would be "shown what South Sudan is doing with regard to peace".
Power said there were "very worrying reports" more weapons and arms were being brought into South Sudan for a fresh offensive, warning there was "no military solution" to end the conflict.
"Fifty thousand children under five are at risk of dying by malnutrition in the coming months, and around half of this country's population is facing grave food insecurity."
The United Nations has said the South Sudan food crisis is the "worst in the world", with aid workers warning of famine within weeks if conflict continues.
US Secretary of State John Kerry yesterday accused both sides of failing to commit to the peace process, a day after they missed a key deadline to forge a unity government.