Kenyan forces were on Sunday operating deep inside Somalia with orders to make sure there were no al-Shabaab extremists within 100 kilometres of their common border.
Units from the Kenya Army are understood to have crossed into Somalia at the northern points of Liboi and Mandera with instructions to fight their way into the strife-torn country and create a buffer zone to ensure that insurgents do not launch attacks against Kenya.
The action follows a spate of kidnappings in Kenya by suspected members of al-Shabaab that Nairobi said was threatening its territorial integrity and its economy which leans heavily on tourism.
Kenya Air Force and the Navy would be sent out as the need arises, security sources said.
Kenya soldiers actually crossed into Somalia days before the announcement by Internal Security minister George Saitoti and Defence minister Yusuf Haji of the offensive into foreign territory. (READ: Kenya declares war on Al Shabaab militants)
Kenya's Internal Security permanent secretary Francis Kimemia said security forces had drawn up strategies to defeat al-Shabaab in their own land as it invoked article 51 of the UN charter which allows for self-defence.
“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.
In an apparent response, al-Shabaab warned Kenyan soldiers that they would feel the "pain of bullets."
"Kenya violated the territorial rights of Somalia by entering our holy land, but I assure you that they will return disappointed, God willing," said Sheikh Hassan Turki, a senior al-Shabaab leader, on Sunday. (READ: Al Shabaab warning as Kenya troops enter Somalia)
"Mujahideen fighters will force them to test the pain of the bullets."
A top Somali diplomat said the involvement of Kenyan soldiers on Somali soil could make "the situation worse".
The first secretary of Somalia's mission to the United Nations, Omar Jamal, said the reports of Kenyan soldiers crossing the border had not been officially confirmed but said he was afraid the involvement of Kenyans in Somalia could make the situation worse.
"We understand the Kenyan concerns very well," he said.
"However if any action is to be taken... the Somali government has to be on the same page, the Somali government has to be informed, the Somali government has to know exactly in many details what is going on, otherwise it will be a different story."
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," said Mr Kimemia.
"And it also allows you to pursue those who have hit and ran away."
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. (READ: Massive hunt as gunmen kidnap aid workers)
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 where he branded al-Shabaab rebels “the enemy” and vowed to attack them “wherever they will be.”
Observers said the Kenyan military was probably better armed than at any time in its recent history.