The federal government is advising Canadians in South Sudan to leave immediately amid violence that has killed hundreds of people.

Foreign affairs issued an advisory Saturday evening saying Canadians in the African country should leave and is advising Canadians not to travel there.

"You are urged to leave now by commercial means, as it may become increasingly difficult to do so if the situation deteriorates further," an advisory on the Foreign Affairs website says.

"The Juba (South Sudan capital) airport is open, and tickets for commercial flights are available."

Federal government sources say about 100 Canadians are registered with the government as being in the African country.

Rick Roth, a spokesman for Foreign Affairs Minister John Baird says Canada is deeply concerned about the violence.

"Canada calls for an immediate stop to the fighting in South Sudan and expresses its strong support for the efforts of the UN mission and the African Union to help the parties resolve the current conflict through dialogue," Roth said in an email.

UN helicopter attacked

The travel advisory came after gunfire hit three U.S. military aircraft trying to evacuate American citizens in a remote region of South Sudan that on Saturday became a battle ground between the country's military and renegade troops, officials said. Four U.S. service members were wounded in the attack in the same region where gunfire downed a UN helicopter the day before.

The U.S. military aircraft were about to land in Bor, the capital of the state of Jonglei and scene of some of the nation's worst violence over the last week, when they were hit. The military said the four wounded troops were in stable condition.

The U.S. military said three CV-22 Ospreys — the kind of aircraft that can fly like a helicopter and plane — were "participating in a mission to evacuate American citizens in Bor." A South Sudan official said violence against civilians there has resulted in bodies "sprinkled all over town."

The bodies are sprinkled all over the town. - Michael Makuei Lueth, South Sudan information minister

"After receiving fire from the ground while approaching the site, the aircraft diverted to an airfield outside the country and aborted the mission," the statement said. "The injured troops are being treated for their wounds." It was not known how many U.S. civilians are in Bor.

After the aircraft took incoming fire, they turned around and flew to Entebbe, Uganda. From there the service members were flown to Nairobi, Kenya aboard a U.S. Air Force C-17 for medical treatment, the statement said.

Unannounced visit

An official in the region who insisted on anonymity to share information not made public said the Americans did not tell the top commander in Bor — Gen. Peter Gadet, who defected from the South Sudan military this week — that they were coming in, which may have led to the attack. The U.S. statements said the gunfire was from unknown forces.

South Sudan's military spokesman, Col. Philip Aguer, said that government troops are not in control of Bor, so the attack on the U.S. aircraft has to be blamed on renegade soldiers.

"Bor is under the control of the forces of Riek Machar," Aguer said.

The U.S. Embassy in Juba said it has evacuated at least 450 Americans and other foreign nationals from Juba this week and had hoped to begin evacuations from Bor. The U.S. Ospreys were hit one day after small arms fire downed a UN helicopter in the same state.

Civilians fleeing violence seek refuge at the UN compound in Bor, capital of Jonglei state, in South Sudan. (Hailemichael Gebrekrstos/The Associated Press)

The UN on Friday sent four helicopters to extract 40 UN peacekeepers from a base in Yuai, also in Jonglei, U.N. information officer Joe Contreras said. One helicopter was fired upon and executed an emergency landing in Upper Nile state, he said. No casualties occurred during the incident.

South Sudan's information minister, Michael Makuei Lueth, said that South Sudanese ground troops, backed by the country's air force, are fighting rebels in Bor, an effort to retake the state capital they lost earlier this week.

"There is fighting going on in Bor town, yes, because since morning they have continued to attack the civilian population," he said, talking about renegade troops. "They have gone as far as not respecting the UN compound."

He said fighting started early Saturday after reports came in that rebels there were shooting indiscriminately at civilians.

"The bodies are sprinkled all over the town," he said. No death toll could be estimated, he said.

Attempted coup

South Sudan President Kiir, an ethnic Dinka, said this week that an attempted coup triggered the violence now pulsing through South Sudan. He blamed the former vice-president, Machar, an ethnic Nuer. But officials have since said a fight between Dinka and Nuer members of the presidential guard triggered the initial violence late Sunday night. Machar's ouster from the country's No. 2 political position earlier this year had stoked ethnic tensions.

The violence has killed hundreds and has world leaders worried that a full-blown civil war could ignite in South Sudan. The south fought a decades-long war with Sudan before a 2005 peace deal resulted in a 2011 referendum that saw South Sudan break away from the north, taking most of the region's oil wealth with it.

Lueth described Machar as "the rebel leader," saying the forces that control Bor believe they are fighting on his behalf. Machar's whereabouts remain unknown, but he has said in recent interviews that he is in hiding somewhere in South Sudan.

South Sudan, the world's newest country, is threatened by rapidly escalating ethnic violence. (The Associated Press)

An International Crisis Group expert on South Sudan told The Associated Press on Friday that rebels have taken control of at least some of South Sudan's oil fields, an issue that could bring Sudan into the conflict. South Sudan's oil flows north through Sudan's pipelines, providing Khartoum with much needed income.

The U.N. Security Council on Friday said the weeklong violence could affect neighbouring countries and the entire region.

Kenya announced it was sending in Kenyan troops to evacuate 1,600 Kenyans stranded in South Sudan, many of them in Bor.

U.S. President Barack Obama earlier this week dispatched U.S. troops to help protect the U.S. Embassy in the capital, Juba. The U.S. Embassy organized at least five emergency evacuation flights to help U.S. citizens leave the country. Other countries like Britain, Germany and Italy also helped citizens evacuate.

Mediators from East Africa continued to try to help negotiate a peace. Ethiopia's Foreign Ministry said Saturday that they have held "productive" talks with Kiir and that consultations were continuing. Kiir has agreed to "unconditional dialogue" to try to stop the violence.