Kenyan forces were on Sunday operating deep inside Somalia with orders to make sure there are no al Shabaab extremists within 100 kilometres of the border.
Units from the Kenya Army are understood to have crossed into Somalia at Liboi and Mandera with orders to fight their way into the lawless country and create a buffer zone to ensure that insurgents do not launch attacks against Kenya.
Kenya Air Force and the Navy will be sent out as the need arises, security sources told the Nation.
Soldiers actually crossed into Somalia days before the announcement by Internal Security Minister George Saitoti and Defence Minister Yusuf Haji.
Internal Security permanent secretary Francis Kimemia said security forces had drawn up strategies to defeat al Shabaab in their own land.
"How it will be done, the number of troops involved and where they will strike remains a preserve of the military. We can't give information that would be useful to the enemy," he said.
Witnesses have reported military trucks at border points and military choppers in the air. Foreign troops are already on the ground in Somalia, mainly from Uganda and Burundi, under the African Union.
The Federal Transitional Government, the weak authority in Somalia backed by the international community, also has troops trained in Kenya and Uganda and paid for by western donors.
"Now we can't wait for the TFG or Amisom. Article 51 of the UN charter allows us to pursue them. It allows you to hit anybody who hits you or is planning to hit you.
"And also allows you to pursue those who have hit and ran away," Mr Kimemia said.
Kenya has been under sustained provocation from al Shabaab for months, with the al-Qaeda-linked group raiding across the border and, last Thursday, kidnapping two Spanish aid workers at the Daadab refugee camp.
French woman Marie Dedieu was kidnapped on Manda Island on October 1 while Mrs Judith Tebbutt, a Briton, was abducted and her husband David shot dead at Kiwayu resort in Kiunga in September.
In all incidents, those captured were taken to Somalia and are being held in al Shabaab bases. Prof Saitoti announced the decision to send troops to Somalia in a press conference on Saturday.
He branded Somalia's al-Qaeda-inspired al Shabaab rebels "the enemy" and vowed to attack them "wherever they will be."
The military is probably better armed than at any time in its recent history.
Last year, Kenya was reported to have spent Sh27 billion on combat aircraft, helicopter gunships, grenade launchers and other weapons and was Africa's fourth largest military spender after South Africa, Angola and Sudan.
Kenya can also raise a reasonably large army -- last year the Central Intelligence Agency estimated that 20 million Kenyans were available for military service. Out of these, about 12 million men and women were fit for service.
Military sources traced the upsurge in militia activity to August 12, when al Shabaab vanished from Mogadishu overnight, following a sustained assault by AU and Somalia government forces.
It appears that, having lost the war on that front, the extremist group is testing Kenya as a "soft target" for lucrative hostages.
Article 51 of the UN Charter, which Kenya is relying on to justify sending its troops across the border, says:
"Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.
"Measures taken by Members in the exercise of this right of self-defence shall be immediately reported to the Security Council and shall not in any way affect the authority and responsibility of the Security Council under the present Charter to take at any time such action as it deems necessary in order to maintain or restore international peace and security."
Set up security posts
Meanwhile, Kenya needs to develop a long-term strategy to secure all its borders, the chairman of the Parliamentary Committee on Defence has said.
Mr Adan Keynan, who is also the MP for Wajir West, said the military should start this process by setting up security posts at intervals of 20 kilometres along the border with Somalia.
Mr Keynan said the action against the al Shaabab would be an opportunity for the army to show Kenyans how well its peacetime army can do its job.
"We have one of the most professionally run disciplined forces in Africa," said Mr Keynan, adding, the Army would be judged harshly if it does not succeed in pushing away the militia.
Government spokesman Alfred Mutua said the pursuit of the Somalia militia group was on. "We will give the details tomorrow (Monday)," he said.
An AFP reporter close to the border witnessed large numbers of troops as well as military planes and helicopters overhead.
Several witnesses reported heavy troop movement in Kenya's border regions, with truckloads of soldiers heading towards the frontier.The kidnappings at the Coast have dealt a major blow to the tourism industry.
Speaking when he arrived in the country after an international trip, Tourism Minister Najib Balala said the insecurity caused by Somali militias requires an international solution.