Hundreds of Kenyan troops ventured deep into Somalia to hunt Al Shebaab fighters blamed for a spate of kidnappings, prompting the Al Qaeda group to threaten 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 Al Shabaab militia and confirmed that it had sent its army across the border.

Kenyan soldiers were reported near the village of Qoqani, some 80 kilometres inside southern Somalia, an Al Shabaab 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."

"The Kenyan forces have crossed about 100 kilometres deep into Somalia and in some cases their military aircraft have bombed inside Somalia. If they continue this way, they will regret and feel the consequences back home," Al Shabaab spokesman Sheikh Ali Mohamud Rage told reporters at a press conference.

"Kenya has peace, its cities have tall buildings and business is booming there, while Somalia is in chaos. 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.

Sorry, this audio has expired Kenya strikes back at militants in Somalia ( Ginny Stein )

Kenya confirmed on Sunday its forces had crossed the border into war-torn Somalia to fight hardline Al Shabaab forces they accuse of kidnapping foreigners.

Poised to fight

Hassan Turki, a senior southern Al Shabaab leader, immediately promised that his men would force the Kenyan troops "to test the pain of the bullets".

Al Shabaab fighters were reported on Monday to be boosting defences and sending "hundreds" of fighters towards Kenyan and government positions.

"I saw around 50 trucks and pickup trucks mounted with machine guns, with hundreds of fighters heading towards the Kenyan border," Abdi Jumale, a resident in the Al Shabaab-held port of Kismayo said.

Al Shabaab commanders confiscated at least 100 trucks late Sunday from the Lower Shabelle region, outside the capital Mogadishu, to transport gunmen southwards towards the battle zones, witnesses said.

Five Kenyan soldiers died when their military helicopter providing air cover for ground troops crashed late on 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 Al Shabaab "wherever they will be".

In the past five weeks a British woman and a French woman have been abducted from beach resorts in two separate incidents, dealing a major blow to Kenya's tourism industry.

On Thursday, two Spanish aid workers were seized by gunmen from Kenya's crowded Dadaab refugee camp, the world's largest with some 450,000 mainly Somali refugees.

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.

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

Dadaab in Kenya's north-east is the world's biggest refugee camp, and is receiving a growing stream of new arrivals from famine-struck Somalia. ( AFP: Tony Karumba )

Taking action

Jerry Rawlins, the former president of Ghana and the African Union representative for Somalia, says Kenya has little choice but to take action after a series of abductions.

"Kenya's been trying to hold back for all this long. But the last report that I received is that she's had enough and is actually moving in," he said.

Kenya says it has the support of Somalia's transitional government, a body which falls far short of universal support in Somalia.

The Kenyans, whose forces are the latest to risk their lives in what has proved a deeply intractable conflict, say there is support for now for their government's decision.

"It was high time for the government to show the Al Shabaab, not the Somali people, there are good Somali people," Nairobi resident Jonathan Cheres said.

"But for Al Shabaab, it's high time to show them that they are serious and we are not going to let you continue with this one. We will capture you and make sure that we stop your activities."

This is not the first time that Kenyan security forces have intervened in Somalia.

Small contingents of Somali troops have crossed the border before, but usually those actions were covert with the government covering up casualties.

This time Kenya has made its intentions very clear and very public. They want Al Shabaab gone from its borders and they are willing to put lives on the line to do it.

ABC/wires