"Both parties need to put the interest of South Sudan above their own," Kerry told reporters in Jerusalem.
"Negotiations have to be serious, they cannot be a delay, a gimmick," in order to win military advantage, he said, adding that leaders must have "courage, resolve and clear intent to find a political solution."
Face-to-face peace talks between South Sudan's warring factions were due to begin in earnest Sunday, with artillery fire in Juba's government district underlining the risk of a slide into all-out civil war.
The talks in the Ethiopian capital Addis Ababa are aimed at ending three weeks of fighting that are feared to have killed thousands in the world's newest nation.
The conflict erupted on December 15, pitting army units loyal to President Salva Kiir against a loose alliance of ethnic militia and mutinous army commanders nominally headed by his rival, former vice president Riek Machar.
Aid workers have stepped up warnings of a worsening crisis for civilians affected by the conflict in the landlocked country of almost 11 million people.
The army continued Saturday to battle rebels in a bid to wrest back the strategic town of Bor, capital of Jonglei, one of the country's largest states.
There were reports of intense battles involving tanks and artillery on the outskirts of Bor, which has already changed hands three times since fighting began.
The US embassy in South Sudan ordered a further pullout of staff on Friday because of the "deteriorating security situation", although Washington -- a key backer of the fledgling state -- insisted it remains committed to ending the violence.
Fighting started in oil-rich but impoverished South Sudan when Kiir accused Machar of attempting a coup.
Machar denied this, in turn accusing the president of conducting a violent purge of opponents.