JUBA, South Sudan — Some South Sudanese, including those with dual U.S. citizenship, are not being allowed to leave war-torn South Sudan, even as the United States, India and others continued Thursday to evacuate their citizens while a fragile cease-fire appeared to hold.

The reports that South Sudan's government is checking the political ties of people, especially of men, who are seeking safety have raised fears of further violence in a country trying to heal from civil war.

An Associated Press reporter at the airport in the capital, Juba, saw local authorities refuse about 20 dual South Sudanese-U.S. citizens from leaving the country, despite the presence of U.S. Embassy staff.

The State Department acknowledged that some people had been barred from boarding a chartered flight.

"In a July 14 security message, the U.S. Embassy advised U.S. citizens that the government of South Sudan is scrutinizing the travel documents of dual nationals, with an apparent focus on South Sudanese government and political affiliation, particularly of male dual nationals," spokeswoman Elizabeth Trudeau said.

In a statement, Amnesty International said it had received reports from two charter airline companies that "National Security Service officers have ordered them not to carry South Sudanese nationals, particularly men."

The London-based rights group called the restrictions "totally unacceptable" and called for safe passage for civilians.

Other South Sudanese trying to flee the country by road have reported being turned back from the border.

"We definitely hope that people who wish to leave South Sudan, regardless of their nationality, for their own safety are able to do so without hindrance," said the spokesman for U.N. Secretary-General Ban Ki-moon, Stephane Dujarric.

A convoy of Ugandan troops moving into South Sudan's capital for evacuations was ambushed by gunmen who were repulsed following an exchange of fire, Lt. Col. Paddy Ankunda, the Ugandan military spokesman, said. Three Ugandan soldiers were hurt, he said.

Other countries, including India, were arranging flights for their citizens. Germany's foreign office said those evacuated on Wednesday included three wounded Chinese peacekeepers from the U.N. mission in South Sudan. A fourth wounded Chinese peacekeeper was airlifted Thursday to Uganda.

In Juba, the U.N. peacekeeping mission and U.N. agencies were preparing for the "temporary relocation of non-critical staff from Juba," Dujarric said. He did not say how many people that would include.

Concerns grew about abuses committed during the chaos, which the U.N. says has displaced 42,000 civilians.

Dujarric said reports include "allegations of a killing of at least one South Sudanese national working for an international NGO, as well as rapes, including of an international NGO staff. U.N. staff members have also been assaulted."

The World Food Program said it was outraged by the looting of its main warehouse in Juba, which had held more than 4,500 metric tons of food as well as trucks, generators and other supplies for countrywide operations.

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Associated Press writers Michael Astor at the United Nations and Matthew Lee in Washington contributed.