MOGADISHU, Somalia -- Troops launched an amphibious assault before dawn Friday on the al-Qaida militant group al-Shabab's last stronghold in Somalia.
Other African Union forces were traveling overland to link up with the joint Kenyan-Somali force in the port city of Kismayo.
The commander of the U.N-backed African Union troops, Lt. Gen. Andrew Gutti, said the aim was to "liberate the people of Kismayo to enable them to lead their lives in peace, stability and security."
Col. Cyrus Oguna, the Kenyan military's top spokesman, said the attack met minimal resistance, but al-Shabab denied that the city had fallen and said fighting was taking place.
Oguna told The Associated Press that al-Shabab, which formally merged with al-Qaida in February, had incurred "heavy losses" but that Kenyan forces have not yet had any injuries or deaths.
"We came from the beach side and we're moving towards the main city. Our surveillance aircraft are monitoring every event taking place on the ground," he told Reuters.
"For now, we're not everywhere. We've taken a large part of it without resistance, I don't see anything major happening," he said.
Residents in Kismayo, a city of about 193,000 people, contacted by The Associated Press said that Kenyan troops had taken control of the port, but not the whole city.
"Al-Shabab fighters are on the streets and heading toward the front line in speeding cars. Their radio is still on the air and reporting the war," resident Mohamed Haji told The Associated Press. Haji said that helicopters were hitting targets in the town in southeastern Somalia.
At an international one-day summit Britain's Prime Minister David Cameron said the world would "pay a price" if it fails to help Somalia overcome terrorism, piracy and starvation. ITV's Lee Comley reports.
Another resident, Ismail Suglow, told Reuters that he could hear shelling from the ships and that the rebels were responding with anti-aircraft guns.
"We saw seven ships early in the morning and now their firing looks like lightning and thunder. Al-Shabab have gone towards the beach. The ships poured many AU troops on the beach," he added.
On Thursday, residents said planes had dropped leaflets on Kismayo warning civilians to evacuate within 24 hours, Reuters reported. More than 10,000 residents fled Kismayo in the last several weeks.
Resident Faduma Abdulle said Friday that she is now leaving too.
She said al-Shabab made an announcement on its radio station Friday to trick residents into moving toward the invading troops.
"They told residents through their radio to loot a Kenyan ship that washed up on the coast, but instead the residents who rushed there were attacked by helicopters," she said. "Some of them have died but I don't know how many. The situation is tense and many are fleeing. It's a dangerous situation."
A U.S. military spokesman, Lt. Cdr. Dave Hecht, said the U.S. Africa Command, known as AFRICOM, is closely monitoring the situation but that "we are not participating in Kenya's military activities in the region."
Militants: Taking city not 'a piece of cake'
Al-Shabab said it would not give up Kismayu easily.
"Going into Kismayo is not a piece of cake. We are still fighting them on the beach where they landed," Sheik Abdiasis Abu Musab, al-Shabab's spokesman for military operations, told Reuters on Friday. "For us, this is just the beginning, our troops are spread everywhere."
Oguna said the assault is part of a four-prong attack involving Kenyan forces currently in villages outside Kismayo.
The amphibious assault landed between 10:30 p.m. Thursday and 2 a.m. Friday local time (3:30 p.m. and 7:30 p.m. Thursday ET) , he said. Some of the troops had night-vision goggles, he said.
African Union troops pushed al-Shabab out of Mogadishu in August 2011, ending four years of control of the capital by the fighters.
The Ugandan and Burundian troops that make up the bulk of the African Union force in Mogadishu have slowly been taking control of towns outside of Mogadishu.
The expanding control by AU troops sent al-Shabab fighters fleeing south toward Kismayo, north to other regions of Somalia and across the Gulf of Aden to Yemen, according to American and African Union officials.
Al-Shabab still holds sway across many small, poor villages of southern Somalia. The loss of Kismayo would be significant.
The militants taxed goods coming into its port. Al-Shabab lost its major source of financing last year when it was pushed out of Bakara market in Mogadishu, where it also charged taxes.
The march toward Kismayo by the Kenyan forces has been nearly a year in the making.
Kenyan troops entered Somalia last October after a string of kidnappings inside neighboring Kenya, including of Westerners in and around the beach resort town of Lamu, which is also seeing the construction of a new port and could one day be final point of a new oil pipeline from South Sudan.
Kenyan forces were bogged down by rain and poor roads for months but have making slow and steady progress toward Kismayo in recent weeks.
The Associated Press and Reuters contributed to this report.
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