MOGADISHU (AFP) -- Several thousand residents poured back into Mogadishu on Monday after a four-day calm, but an African Union commander warned that fighting could erupt again and a humanitarian crisis still looms.

Four days after Islamist insurgents and clan fighters melted away in the face of heavy fire by Somali-backed Ethiopian troops, AU troop commander Lieutenant Katumba Wamala cautioned that the battle was not yet won.

"It's not yet a time to celebrate," said Wamala, in charge of 1,500 Ugandan troops, the first contingent of a peacekeeping force struggling to get off the ground.

Wamala warned that insurgent fighters now hiding in the city could still pose a threat.

Meanwhile, renegade Somali leaders in Eritrea said the insurgents had changed tactics but would strike again.

"The resistance fighters are changing their strategy from face-to-face conflict to hit-and-run attacks," said top Islamist leader Sheikh Sharif Sheikh Ahmed and former parliament speaker Sheikh Sharif Hassan Aden in a joint statement.

"The resistance will never give up its mission to fight the invading troops," they warned, also naming the AU peacekeepers as targets.

Ethiopian forces have been battling Islamist fighters and clan militia since the beginning of the year when they backed Somali government soldiers to oust an Islamist movement from the capital and south and central Somalia.

After an Ethiopian offensive on Thursday put an end to nine days of heavy clashes, in which around 400 died, some of the estimated 400,000 displaced civilians began returning home from squalid camps on the outskirts of the city.

At least 4,000 Mogadishu residents, aboard trucks and on foot, streamed back to the shattered capital on Monday, according to elders from the dominant Hawiye clan.

Meanwhile, tens of thousands remained displaced, lacking basic necessities, outside the city.

But for those who returned, the struggle was not over.

"What we can see is total destruction. Hell happened in our neighborhood," said Mohamed Juke Ali, resident of northern Mogadishu where some of the worst fighting took place, after returning to the ruins of his former home.

"From my visit around the city, I have realized that there is a looming humanitarian catastrophe," Wamala told a press conference, adding that civilians were in dire need of water, food, medicine and shelter.

"I came back with my children and husband. We are now in our house and we cannot hear any explosions, but the problem is that we are starting from zero," Hasina, a 35-year-old mother of three told AFP.

Electricity and water supplies have been cut by 75 percent, Hadi Ali Yusuf, a human rights activist, told AFP.

Many people said they had no choice but to use contaminated water supplies.

"My child is suffering from diarrhoea and I think its the water we are using," said Naji Abdulahi Salah, a resident of Tawfiq area. "We used to get water from taps before the fighting but now we are using shallow wells in the neighborhood and they are not clean water."

The UN humanitarian coordinator for Somalia, Eric Laroche, said there had been some 17,000 cases of diarrhoea and cholera in Somalia since January.

"Thirty-four percent of cholera cases come from Mogadishu, and those who have fled are going to spread the virus," he said. "We are heading into a wall. We're heading toward catastrophe."

The UN Security Council on Monday urged all warring sides in Somalia to agree a ceasefire and deny safe haven to extremist elements while pressing for increased humanitarian aid for the battered country's population.

Meanwhile, a landmine killed an unknown number of Ethiopian troops as they were traveling in a truck on a road between the Somali capital Mogadishu and the south central town of Baidoa, police said.

"The (Ethiopian) forces told us they suffered deaths and casualties, but they refused to give us the exact details," local police chief Mohamed Hassan Ahmed told AFP.