The United States has denounced a violation of the South Sudan ceasefire deal negotiated in January after weeks of bloodshed which left thousands dead.
Fighting between the army and rebels on Tuesday in the northeastern town of Malakal appears to have been the heaviest since the government of President Salva Kiir and rebels loyal to former vice president Riek Machar signed the truce.
"We are deeply concerned by reports of extensive fighting in and around Malakal," State Department spokeswoman Marie Harf told reporters on Wednesday at the daily press briefing.
Calling it a "blatant violation of the cessation of hostilities agreement" signed January 23 in Ethiopia, Harf condemned "the failure of both parties" to respect the truce.
The US called "on both parties to end any military actions aimed at the other," she added.
She noted that the US special envoy for Sudan and South Sudan, Donal Booth, was in Ethiopia to participate in the difficult peace talks ongoing there.
The South Sudanese army said on Wednesday it had lost contact with its troops in Malakal, a strategic oil hub, where a rebel assault the night before had reignited the conflict after a month-long break.
South Sudan is the world's youngest nation, having won independence from Khartoum less than three years ago. The latest fighting erupted in the capital Juba on December 15 but quickly spread across the country.
The unrest has already killed thousands and displaced some 900,000.
The United States, which was the prime backer of South Sudan's 2011 independence from Sudan, has increased pressure over the past two months on the combatants to avoid a blow-out in the young nation.
