JUBA, South Sudan — The main opposition group in South Sudan demanded Monday an international probe into alleged rights abuses in recent fighting that saw government troops target areas loyal to the group’s longtime leader, Riek Machar, who is under house arrest.

Machar, the country’s vice president whose political rivalry with President Salva Kiir has repeatedly threatened to tip South Sudan back into civil war, is accused of subversion.

Since March, fighting has engulfed the north, where government troops battled a rebel militia known as the White Army, widely believed to be allied with Machar, with dozens killed. The rebels overran an army base in the town of Nasir, a Machar stronghold. Government troops responded with airstrikes and also attacked opposition forces’ barracks outside the capital, Juba.

Spokesman Pal Mai Deng from Machar’s Sudan People’s Liberation Movement-In-Opposition said the international community should investigate “airstrikes using chemical weapons” in areas such as Nasir.

He did not elaborate. The statement came after Human Rights Watch last week reported the use of air-dropped incendiary weapons by government forces that have “killed and horrifically burned dozens of people, including children, and destroyed civilian infrastructure in Upper Nile state.”

The government, which has ordered civilians to leave Nasir, couldn’t immediately be reached for comment. Along with Machar’s house arrest, many of his allies have also been detained.

Incendiary weapons inflict burns on victims, but they also cause fires that can indiscriminately destroy civilian property, the New York-based rights group said.

“Use of these weapons in populated areas violates international humanitarian law, and if done with criminal intent, constitutes a war crime,” HRW said.

The Nasir fighting threatens a 2018 peace deal between Machar and Kiir to end a five-year civil war in which more than 400,000 people were killed. Machar has served as the country’s first vice president in a national unity government, even though his political group opposes Kiir.

The political rivalry between the two men is widely seen as a major obstacle to peace in South Sudan. Kiir angered Machar’s group earlier in the year by firing officials seen as loyal to Machar, who claims Kiir is endangering the peace agreement through “persistent violations.”

There were high hopes for peace and stability after oil-rich South Sudan gained independence from Sudan in 2011. But the country slid into civil war in December 2013 largely based on ethnic divisions when forces loyal to Kiir started battling those loyal to Machar.

Presidential elections, repeatedly postponed, are now scheduled for 2026.