Kenya hunted Shebab militants deep inside Somalia Monday, claiming self-defence for an unprecedented incursion which the Somali insurgents warned would trigger reprisals in the heart of Nairobi.

Backed by aerial bombings and guided by pro-government Somali forces, Kenyan troops moved deeper into southern Somalia, a day after Nairobi declared war on the Shebab militia and confirmed that it had sent its army across the border.

Kenyan soldiers were reported near the village of Qoqani, some 50 miles inside southern Somalia, a Shebab stronghold.

"The Kenyan troops supported by tanks and other military vehicles have taken up positions near Qoqani," said Saleban Mohamed, an elder in a nearby village.

"I saw around 32 trucks and tanks, with hundreds of troops," he added.

"The Kenyan soldiers are heavily armed and they have started digging trenches near Qoqani," Abdulahi Sayid Adam, another witness said.

The last time one of Somalia's neighbours unilaterally invaded was in late 2006 when Ethiopia started an occupation that lasted two years and spurred the formation of the Al Qaeda-inspired Shebab insurgency.

"Kenya has peace, its cities have tall buildings and business is booming there, while Somalia is in chaos," Shebab spokesman Sheikh Ali Mohamud Rage told reporters at a press conference.

"If your government ignores our calls to stop its aggression on Somali soil, we will strike at the heart of your interests," he said, addressing the Kenyan population.

Kenya confirmed Sunday its forces has crossed the border into war-torn Somalia to fight Shebab forces they accuse of kidnapping foreigners, a claim the insurgent group has so far rejected.

Residents said Shebab commanders confiscated civilian vehicles to send reinforcements from the outskirts of Mogadishu to the north and from the port city of Kismayo to the south.

"Many trucks were taken by Shebab from civilians to transport fighters, they are heading towards the Juba regions," a witness in the Shebab-held Afgoye district near Mogadishu told AFP, asking not to be named for security reasons.

"They have collected all weapons near Afgoye, and gathered hundreds of young fighters to face their enemy," another witness said.

Five Kenyan soldiers died when their military helicopter providing air cover for ground troops crashed late Sunday due to reported "technical problems" near Liboi, just inside Kenya's border, army spokesman Emmanuel Chirchir said.

The assault came after Kenya's Internal Security Minister George Saitoti vowed to attack the Shebab "wherever they will be."

"The Kenya government is taking robust measures to protect and preserve the integrity of the country," Defence Minister Yusuf Haji said Monday, dubbing the invasion Operation Linda Nchi -- or "Defend our Country."

In the past five weeks a British woman and a French woman have been abducted from beach resorts in two separate incidents.

On Thursday, two Spanish aid workers were seized by gunmen from Kenya's crowded Dadaab refugee camp.

It was unclear how long Kenyan troops planned to stay in Somalia but Nairobi had been under growing pressure to take action and attempt to restore confidence that it could safely host tourists and one of the world's largest aid communities.

The Shebab had long refrained from carrying out attacks inside Kenya, which observers argued was useful to them as a logistical and financial base.

Kenya's assault prompted mixed reaction in its sizable Somali community in Nairobi's Eastleigh district.

"Fixing the security by ending the Shebab is a good idea, but I think military attacks like this could make the situation worse," said Ahmed Ali, who works in an electronics store.

"Kenya is strong but they face a hard battle in Somalia, and the fight will not be easy," warned unemployed Somali refugee Mohamed Ali.

While Kenyan troops have frequently been reported to have crossed the long porous border in recent years, Nairobi had never confirmed any involvement and was always cautious to minimise its exposure to reprisals.

In July 2010, the Shebab group claimed responsibility for suicide attacks in central Kampala that killed at least 76 people in what it said was retaliation for Uganda's leading role in the Mogadishu-based African peace force.