DAKAR, Senegal — A major surge in fighting in Sudan has taken a searing toll on civilians, killing hundreds of people in aerial bombings and revenge attacks in the past week, as Africa’s largest war shifts into a higher gear after the end of seasonal rains.

Territory has changed hands; a prominent commander has switched sides; and retreating fighters have sexually assaulted, kidnapped and killed villagers as they have moved through contested countryside, according to activists, democracy groups and accounts on social media.

A military cargo plane slammed into the desert in the western region of Darfur, with at least two Russian crew members on board, offering direct evidence of the growing role of foreign contractors in the fighting.

And Sudan’s military, after losing control of vast areas of Sudan, has finally seemed to regain the advantage over the Rapid Support Forces, the powerful paramilitary group that it has been battling for the past 18 months. Both sides face a barrage of war crimes accusations from the United States and rights groups, although only the RSF has been accused of ethnic cleansing.

“The fighting season has just restarted, and both sides want to jostle for an early advantage,” said Kholood Khair, the founding director of Confluence Advisory, a policy think tank.

The escalating violence comes against a vast tableau of suffering. More than 10 million have been forced from their homes, famine is raging, and diseases like cholera and dengue fever are rapidly spreading.

Diplomatic efforts to end the war have stalled, with neither side showing much willingness to compromise on anything, much less reach a cease-fire. Support is growing among activists, peacekeeping experts and human rights groups for the United Nations to deploy a mission to protect civilians, but many are skeptical such a force could be mustered.

“We fear it is on the road to becoming a repeat of the 1994 Rwanda genocide,” Roméo Dallaire, who led the U.N. mission in Rwanda during the genocide, wrote in Foreign Policy on Friday.

In the past week, the army, led by Gen. Abdel Fattah Burhan, captured territory in the breadbasket states of El Gezira and Sennar, which the paramilitary forces seized starting last December.

Those gains came in part thanks to the defection of Abu Aqla Kaykal, a local militia leader in El Gezira state who, until recently, was fighting with the RSF. Experts said his defection gave the military a political boost and would cut off the paramilitary group’s ability to recruit from the large Shukria tribe.

By publicizing the defection, Khair said, “the military are trying to assert their claim that they are on the right side” and that their wins are “a victory for the people of Sudan.”

The paramilitaries, who are led by Gen. Mohammed Hamdan Dagalo, launched retaliatory attacks in villages across El Gezira, local activists reported. At least 300 civilians were killed when fighters rampaged through Tambul village, Elmubir Mahmoud, the secretary general of the Al Jazeera Conference, a volunteer group in the state, said in an interview. Gunmen looted homes, took hostages and sexually assaulted women, he said.

Fifty more civilians were killed in a nearby village Friday morning, he said, and 200 were wounded. Entire families fled with nothing but their clothes, he added. Others posted handwritten lists of the dead. The New York Times could not independently verify the lists or the figures.

Video footage and photographs from the area that were shared on social media showed villagers standing over dozens of bodies wrapped in funeral shrouds. The footage and photographs could not be immediately verified.

“The situation is very tragic,” Mahmoud said.

The RSF denied killing civilians, saying those killed had been fighting alongside the military.