SANA, Yemen — Islamist militants consolidated control over a second city in southern Yemen on Sunday, seizing banks, government offices and the security headquarters as government forces responded with mortar fire.

The fall of the coastal city of Zinjibar to self-styled holy warriors who claimed to have “liberated” it from “the agents of the Americans” fed into Western fears that militants sympathetic to Al Qaeda could exploit the breakdown of authority to take control of territory.

Political opponents of Yemen’s embattled president, Ali Abdullah Saleh, portrayed the takeover as a ploy by Mr. Saleh to prove to wavering allies why they needed to keep him in power. While Mr. Saleh, who has faced months of massive street protests demanding his ouster, has frequently warned that militants would take over the country if he left, there was no evidence on Sunday that he had any role in allowing Zinjibar to fall.

The fighting in the south came after a week in which tribal fighting in the capital, Sana, pushed the country to the brink of civil war. That front seemed to quiet on Sunday as the government struck a cease-fire deal with its tribal rivals, bringing relative calm here after days of fierce fighting in which more than 100 people were killed.