Kenyan troops invaded al-Shabab's last stronghold in Somalia, coming ashore in a predawn assault Friday. Other African Union forces were traveling overland to link up with the Kenyan forces in the port city of Kismayo.

Col. Cyrus Oguna, the Kenyan military's top spokesman, said the surprise attack met minimal resistance but al-Shabab denied that the city had fallen and said fighting was taking place. Oguna said that al-Shabab has incurred "heavy losses" but that Kenyan forces have not yet had any injuries or deaths.

Residents in Kismayo 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.

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."

"The operation began five days ago with surgical attacks and gun placement at the jetty and warehouse," Oguna said, adding that Somali national army troops participated in the assault.

An al-Shabab spokesman said on Twitter that the al-Qaida-linked militants still control Kismayo.

"The enemy forces have launched a desperate attack on Kismayo this morning and the mujahedeen forces are resisting their attacks," Sheikh Abdiaziz Abu-Musab said over the militants' radio station in Kismayo.

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, 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.

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 announced false propaganda 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."

The commander of the U.N-backed African Union troops, Lt. Gen. Andrew Gutti, said that more of the soldiers were headed to Kismayo to reinforce those that stormed ashore. He said the aim is to "liberate the people of Kismayo to enable them to lead their lives in peace, stability and security. Operations are ongoing to neutralize targets in Kismayo."