With more than a week of fierce fighting including beheaded bodies in the streets, the battle for the northern Mozambique town of Palma has highlighted the southern Africa country's insurgency and threats to its multibillion-dollar investments.
Here's a look at what is known about the rebel group and the challenges facing Mozambique.
WHO ARE THE REBELS?
They're mostly unemployed young Muslim men from Cabo Delgado, the northernmost province on the country's long Indian Ocean coastline.
For centuries, most people there have been Muslims who traded with Swahili dhow sailors and coexisted with Catholicism brought by Portuguese colonial rulers.
Despite rich natural resources, the province has been one of Mozambique's least developed, with low levels of education, health services, and nutrition.
In recent years some unemployed youths have studied abroad on scholarships from Muslim organizations and locals say many returned preaching a more radical form of Islam. In 2017, violence erupted against government targets by a few small bands, often using machetes to kill police and officials.
The rebels have grown to several hundred, they use motorcycles and are now well-armed with automatic weapons and mortars. Military experts say many weapons come from abroad.
WHAT ARE THEY CALLED?
They are known locally as al-Shabab _ Arabic for 'youth' _ but it seems to be just a handy nickname as they don't have any known affiliation with Somalia's jihadi rebels of the same name.
For a few years, the insurgents didn't appear to be linked to any group, but in 2019, the Islamic State group began claiming responsibility for their attacks, calling them the Islamic State Central African Province.
IS also posts photos and videos of the militants, often standing by the group's black flag. A video posted this week showed them dressed in a mix of camouflage and black shirts and red scarves, and speaking Swahili and some Arabic.
ARE THEY GAINING GROUND?
The number of attacks since 2017 has risen to more than 838, and more than 500 of those have been in the past year, according to the Armed Conflict Location and Event Data project.
More than 2,600 people have been killed. The humanitarian crisis has also dramatically increased, from 90,000 displaced at the start of 2020 to more than 670,000 now, according to U.N. organizations. More than 900,000 people in the area need food aid, according to the World Food Program.
After years of hit-and-run attacks, the rebels captured the port town of Mocimboa da Praia in August and have held it since then. They've attacked smaller towns in the surrounding area.
In one massacre, they beheaded 50 people on a soccer pitch, according to a report confirmed by the Catholic bishop of Pemba, the provincial capital, where hundreds of thousands have fled. The rebels target government offices, kill local officials and rob banks.
HOW IS THE GOVERNMENT RESPONDING?
President Filipe Nyusi's government in Maputo, in the southernmost part of Mozambique, has launched a counterterrorism offensive by the national police and the military.
It also has used a private military organization based in South Africa, the Dyck Advisory Group, which has sent helicopter gunships and other aircraft to find and attack the rebels.
Because the rebels often mingle with civilians, military action is difficult. Atrocities have been committed by all sides _ the rebels, the government forces and the mercenaries _ according to a March 2 report by Amnesty International. The government and the Dyck group deny the charges, saying they are investigating them.
IS MOZAMBIQUE GETTING HELP?
The United States last month declared Mozambique's rebels to be a terror organization and sent special operations forces officers to carry out a two-month training of Mozambique's marines.
Portugal said it's sending 60 officers to provide training and said the European Union is considering military support.
Mozambique is a member of the 16-nation Southern African Development Community, which has been closely watching the instability. The group has had a few meetings on the rebels but Mozambique hasn't yet requested direct military help from neighboring countries, including South Africa and Zimbabwe.
WHAT IS THE ECONOMIC IMPACT?
Rebel violence had caused a suspension of work by the French oil and gas firm Total in January.
On March 24, Total said security had improved enough to allow it to resume, but within a few hours, the rebels attacked Palma, and Total once again evacuated workers from the fortified construction site.
Experts say it will be a long time before stability is sufficiently restored for Total to get back to work. The huge deposits of natural gas are reported to be among the world's largest, and the government was hoping the projects would bring much-needed economic growth.
Exxon also was planning an investment, but that appears to be on hold.
'The whole gas gamble was bet on a promise of security, and Nyusi -- and Mozambique -- lost the bet,' wrote academic Joseph Hanlon in the newsletter Mozambique News Reports and Clippings.
WHAT IS THE OUTLOOK FOR MOZAMBIQUE AND AFRICA?
The rebels have grown in size and organization. Once viewed as a ragtag bunch of dissatisfied youths, their attacks are more strategic and they are spreading their reach over a large part of northern Cabo Delgado.
Military experts say restoring stability will be a long, violent and challenging process. A more long-range solution would be to improve local governments and provide better services and living conditions, according to analysts and military experts.
But that will be difficult, with the rebels already entrenched. Africa's arc of extremism _ from the Sahel region in West Africa, to Nigeria's Boko Haram insurgency in central Africa and al-Shabab's entrenched conflict in Somalia in East Africa _ has a new foothold in southern Africa in Mozambique that will be hard to dislodge.