Gunfire and heavy artillery fire persisted Saturday in parts of Sudan's capital Khartoum, residents said, despite the extension of a cease-fire between the country's two top generals, whose battle for power has killed hundreds and sent thousands fleeing for their lives. Meanwhile, the first operation is underway to free U.S. civilians trapped in Sudan, with several hundred Americans preparing to sail to safety. 

"The U.S. government has taken extensive efforts to contact U.S. citizens in Sudan and enable the departure of those who wished to leave," State Department Spokesperson Matthew Miller said in a statement. "We messaged every U.S. citizen in Sudan who communicated with us during the crisis and provided specific instructions about joining this convoy to those who were interested in departing via the land route."

More than 500 people have been killed and thousands wounded since the fight for the capital began two weeks ago, leading to an exodus from Africa's third largest country. CBS News has confirmed that Americans left the capital of Khartoum on Friday in a convoy of 18 buses, embarking on a drive that took 12 hours to the coast. The plan is to sail them across the Red Sea to Jeddah in Saudi Arabia.

Khartoum, a city of some 5 million people, has been transformed into a front line in the grinding conflict between Gen. Abdel Fattah Burhan, the commander of Sudan's military, and Gen. Mohammed Hamdan Dagalo, who leads the powerful paramilitary group known as the Rapid Support Forces, which has dashed once-euphoric hopes of Sudan's democratic transition.

Foreign countries continued to evacuate diplomatic staff and nationals while thousands of Sudanese fled across borders into Chad and Egypt. Up to 20,000 refugees — mostly women and children — have crossed over the western border to Chad, the United Nations said, a country that has struggled for stability in the aftermath of its own coup two years ago.

• 2 Sudan generals are at war with each other. Here's what we know.

Those who escape the fighting in Khartoum face more obstacles on their way to safety. The overland journey to Port Sudan, where ships then evacuate people via the Red Sea, has proven long and risky. Hatim el-Madani, a former journalist, said that paramilitary fighters were stopping refugees at roadblocks out of the capital, demanding they hand over their phones and valuables.

"There's an outlaw, bandit-like nature to the RSF militia," he said, referring to the Rapid Support Forces. "It indicates they don't have a supply line in place and that could get worse in the coming days."

Airlifts from the country have also posed challenges, with a Turkish evacuation plane hit by gunfire outside Khartoum on Friday.

On Saturday — despite a cease-fire extended under heavy international pressure by another 72 hours early Friday — clashes continued around the presidential palace, headquarters of the state broadcaster and a military base in Khartoum, residents said. The battles sent thick columns of black smoke billowing over the city skyline.

In a few areas near the capital, including in Omdurman, residents reported that some shops were reopening as the scale of fighting dwindled, with both sides seeking to observe a tenuous cease-fire. But in other areas, residents sheltering at home as explosions thundered around them said fighters were going from house, terrifying people and stealing whatever they could find.

Now in its third week, the fighting has left swaths of Khartoum without electricity and running water. Those sheltering at home say they're running out of food and basic supplies. Residents on Saturday in the city of Omdurman, west of Khartoum, said they'd been waiting three days to get fuel — complicating their escape plans.

The U.N. relief coordinator, Martin Griffiths, said that U.N. offices in Khartoum, as well as the cities of Genena and Nyala in Darfur had all been attacked and looted. "This is unacceptable — and prohibited under international law," he said.

Over the past 15 days of pummeling each other, the generals have each failed to deal a decisive blow to the other in their struggle for control of Africa's third largest nation. The military has appeared to have the upper hand in the fighting, with its monopoly on air power, but it has been impossible to confirm its claims of advances.

"Soon, the Sudanese state with its well-grounded institutions will rise as victorious, and attempts to hijack our country will be aborted forever," the Sudanese military said on social media Saturdya.

Many hospitals in Khartoum and across the country have closed.

Few had hope that the conflict would end anytime soon.

"Both parties are digging in," said el-Madani, the former journalist. "This war could go on for a long time."

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• Sudan