MOGADISHU, Somalia — Somalia’s Islamist extremists, Al Shabab, appear to be making a comeback, having seized four towns and attacked a guesthouse in neighboring Kenya, killing 12.

The resurgence of Al Shabab, which is allied to Al Qaeda, could affect Somalia’s plans to hold elections next month and further destabilize what is already one of the world’s most fragile states.

The rebels had steadily lost ground over the past five years, first losing control of the capital, Mogadishu, in 2011 and then being pushed out of Somalia’s other major cities and most towns. This was largely the work of the African Union force of 22,000 soldiers from Ethiopia, Kenya, Uganda, Burundi, and Djibouti, which is supported by the United Nations. The relatively weak Somalia army, with 35,000 troops, also participated.

Al Shabab was reduced to roaming around Somalia’s vast, arid scrubland and staging suicide bombings in Mogadishu and other centers.

But this month, Ethiopia — which has 2,000 troops in the African Union force and an unknown number operating independently in Somalia — pulled forces out of the towns of Halgan, El-Ali, and Mahas in south-central Somalia. Within hours, Al Shabab fighters had seized control of the towns and raised their black flags.

On Wednesday, Ethiopian troops withdrew from a fourth town, Tiyeglow, in the southwestern Bakool region, and Al Shabab retook it.

It is not known how many Ethiopian troops have been pulled out, but analysts say the move was a response to Ethiopia’s need for troops at home to enforce the state of emergency imposed to quell antigovernment protests.

Al Shabab’s swift seizures are worrisome for Somalia, which faces a presidential election in late November.

Somalia’s civilians are bearing the brunt of the withdrawals, with militants executing suspected government collaborators in each location they recapture, say residents.