ABA, Democratic Republic of Congo — As diplomats began another round of peace talks over the civil war in South Sudan, the country’s military forces captured a rebel headquarters, further weakening the fractured rebel movement and prompting a mobilization of soldiers in a neighboring nation.

South Sudan’s army marched Sunday night into the town of Lasu in the country’s southwest, near the border of the Democratic Republic of Congo. The city fell on Monday, according to rebel officials.

Lasu was the southern headquarters of the country’s major rebel group, the SPLA-IO, after the fall of its main base in the country’s northeast this summer.

Ateny Wek Ateny, a spokesman for President Salva Kiir of South Sudan, insisted that the government was adhering to the unilateral cease-fire that Mr. Kiir had declared in May.