By Mading Ngor and Marie-Louise Gumuchian CNN
JUBA, South Sudan (CNN) -- Rebels in South Sudan have taken the town of Bor, government officials said Thursday, as deadly clashes spread in Africa's newest nation.
The town, which lies about 200 kilometers (125 miles) north of the capital, Juba, was under heavy shelling, said Col. Phillip Aguer, an army spokesman. Bor Mayor Mhial Majak Mhial said the town was under rebel control and heavy artillery was in use.
The fighting has been spreading in South Sudan after a reported coup attempt in Juba over the weekend. President Salva Kiir has blamed soldiers loyal to his former vice president, Riek Machar, for starting the violence.
"The government has lost control of Jonglei state to the forces of Col. Machar and his group," government representative Ateny Wek Ateny told CNN, referring to the state where Bor is situated.
The government could not yet confirm the number of fatalities from the violence, but "casualties are in the hundreds, including army forces and civilians," Ateny said.
The government tweeted that its troops had lost control of Bor and that it was doing all it could to "make sure citizens are secure & safe."
Earlier, Joseph Contreras, acting spokesman for the U.N. mission in South Sudan, told CNN the situation in Bor was "still unstable."
The violence, which the government said Wednesday has killed about 500 people, began Sunday evening.
In an interview with the online Paris-based Sudan Tribune published Wednesday, Machar denied Kiir's claims that he was behind an attempted coup, saying: "There was no coup. What took place in Juba was a misunderstanding between presidential guards within their division."
On Thursday, Kiir told reporters he was ready for dialogue "with anyone who is willing," the website said.
The crisis has alarmed South Sudan's neighbors, prompting African nations to send mediators.
The African Union said the team was made up of ministers and high-level officials from Ethiopia, Kenya, Uganda and other countries -- the first major peace initiative since clashes first erupted.
Tensions have been high in South Sudan since Kiir dismissed his entire Cabinet, including Machar, in July. The move further inflamed deep tensions between Kiir's Dinka community and Machar's Nuer community.
U.N. officials have said they are worried the recent fighting is based on ethnic divisions.
Citing reports it had received, Human Rights Watch said Thursday that in the fighting in Juba, South Sudanese soldiers fired indiscriminately in highly populated areas and targeted people for their ethnicity.
According to witnesses and victims, soldiers specifically targeted people from the Nuer ethnic group, it said, adding that it had also received reports that Nuer soldiers may have targeted ethnic Dinka in Juba and in the town of Bor.
"The awful accounts of killings in Juba may only be the tip of the iceberg," Daniel Bekele, Africa director at Human Rights Watch, said in a statement.
"Government officials -- whatever their politics -- need to take urgent steps to prevent further abuses against civilians and quickly deescalate rising ethnic tensions. ... We are deeply concerned that ethnically-based attacks on all sides will lead to revenge attacks and more violence."
It was not immediately possible to independently verify the reports. Government officials have disputed that the fighting is along ethnic lines.
Information Minister Michael Makuei Lueth said Wednesday that people from different states and tribes rose up against the government, which is now fighting back.
Eleven people have been arrested in connection with the foiled coup, according to a statement the South Sudan government posted Thursday. Two of them were from the Nuer community, it said.
Tens of thousands displaced
Up to 100,000 people have been displaced by the fighting so far, according to Defense Minister Gen. John Kong Nyuon.
Many of the displaced people have crossed the Nile River, he said, adding that he feared a humanitarian disaster was unfolding.
On Wednesday, South Sudan's Ministry of Foreign Affairs said order had been restored in the country, adding that government forces were in charge and flights had resumed at Juba International Airport.
U.N. officials have expressed deep concern about the unrest, saying between 15,000 and 20,000 people have taken shelter at the organization's compounds.
Civilians have sought shelter at U.N. compounds in six states across the country, the U.N. Mission in South Sudan (UNMISS) said on its Twitter feed. In Bentiu, in the northern Unity State, it said oil workers also were seeking protection at its compound.
The U.S. and Britain have both sent planes to airlift their nationals out of the country.
U.N. Secretary-General Ban Ki-moon has been reaching out to key leaders seeking a political end to the crisis. The U.N. Security Council has urged all parties to immediately cease hostilities and exercise restraint to prevent the further spread of violence.
South Sudan formally split from Sudan in 2011 after a referendum, following decades of conflict. Numerous armed groups remain active in the oil-rich country.
CNN's Aliza Kassim and Anna Maja Rappard contributed to this report.
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