Clashes spread beyond South Sudan’s capital, Juba, on Wednesday as rival military forces battled in a remote region of the world’s newest nation. The U.S. Embassy evacuated all nonessential American staff members because of the continuing unrest.

Fighting erupted around Bor, the capital of the central state of Jonglei, early Wednesday as troops loyal to former vice president Riek Machar reportedly battled soldiers loyal to the government, led by President Salva Kiir Mayardit. Machar’s forces managed to take control of two areas, according to South Sudanese military officials. The violence has raised concerns that South Sudan, a key oil producer and U.S. ally, could descend into a broader civil war, pitting ethnic groups against each other.

“The situation is tense in Bor,” South Sudanese military spokesman Col. Philip Aguer said by phone. “The fighting took place in areas outside of Bor town. We are still monitoring the situation there.”

The clashes marked the fourth straight day of violence in South Sudan, which separated from Sudan in 2011 after a U.S.-backed peace process ended one of Africa’s longest and most brutal civil wars. Since Sunday, as many as 500 people have been killed, according to the United Nations, after what Kiir has described as an attempted coup by Machar loyalists.

On Wednesday, during a visit to the Philippines, Secretary of State John F. Kerry expressed concern about the spreading violence.

“The United States believes very strongly that all parties should refrain from any actions that could further escalate the tensions,” said Kerry, who visited South Sudan when he was a senator. He called on the government to “respect the rule of law.”

“Political differences need to be resolved by peaceful and democratic means, and those have been hard-fought for,” Kerry said, adding that the State Department is taking steps to guarantee the security of American diplomats still in Juba.

On Wednesday, Kiir told reporters in Juba that he was willing to hold talks with Machar, who is being sought by South Sudanese security forces.

In tweets Wednesday, the U.N. mission in South Sudan reported that clashes had also unfolded Tuesday night near a military base in Torit, east of Juba. More than 1,000 civilians have sought refuge in the U.N. compound in Juba, the United Nations reported.

The fighting in Bor is particularly troubling because it was the site of a 1991 massacre in which ethnic Nuer soldiers loyal to Machar killed hundreds of Dinka, Kiir’s ethnic group. If the conflict intensifies and widens, it could affect humanitarian aid missions across South Sudan, one of the world’s least-developed nations. Neighboring Sudan, which has had recurring disagreements with South Sudan over oil, security and undefined borders, could also exploit the fighting, heightening regional tensions.

A U.S. Embassy official in Juba, speaking on the condition of anonymity because he was not authorized to speak to the media, said that all nonessential embassy staff and other American citizens had been evacuated from Juba on Wednesday. Ambassador Susan D. Page and other essential diplomats were still in the country, the person said, describing the environment in the capital as “calm.”

Later, State Department deputy spokeswoman Marie Harf confirmed the departure of three groups of U.S. citizens and said Page met Wednesday with Kiir “to discuss our concern about the continued violence, increasing death toll and growing humanitarian challenges.” She also raised questions about the arrests of several opposition members, Harf said.

“We continue to urge U.S. citizens to depart South Sudan and will work to arrange for additional transportation as necessary to accommodate demand, taking into account security conditions and availability of regular commercial flights,” Harf said.

The British government also announced Wednesday that it was evacuating some of its embassy staff members and their families.

The United States and its allies have injected billions of dollars into South Sudan in an attempt to transform it into a stable, pro-Western anchor in a region threatened by militant Islam and cross-border terrorism. But since achieving independence, the nation has been plagued by ethnic, tribal and political tensions.

The current tensions began in July, when Kiir fired Machar, along with the entire cabinet, after Machar declared he would challenge Kiir in the next presidential elections, scheduled for 2015. Machar, in an interview published Wednesday in the online, Paris-based Sudan Tribune newspaper, denied that he had attempted to overthrow Kiir this week.

But Aguer said Machar’s loyalists intended to seize power. “They occupied the headquarters of the presidential guard, and that’s when the fighting took place,” said Aguer. “If it was not an attempted coup, what was the point of occupying the headquarters of the presidential guard?”

At least 10 people, including seven former ministers, have been arrested over the incident, and others are being sought for questioning, according to government officials. Some analysts said they were not convinced Machar had staged an attempted coup and attributed the latest violence more to internal tensions that have divided both the government and the military in recent months.

“The problem has escalated fairly quickly,” said Cedric Barnes, director of the Horn of Africa project for the International Crisis Group. “While conditions seem to be improving in Juba, they are deteriorating elsewhere in the country. The risk of civil war is growing.”

Anne Gearan in Washington contributed to this report.