Somalia's al Qaeda-linked Shabaab rebels have called on its operatives in Kenya to launch reprisal attacks as Nairobi vowed to defeat the Islamist insurgents.
Kenya deployed troops against Shabaab positions in southern Somalia 12 days ago, and heavy fighting raged between the two sides on Thursday.
"Mujahidin in Kenya including those who were trained in our military camps must fight inside Kenya," Mukhtar Robow Ali, a top Shabaab military commander told a crowd in a village near Mogadishu.
"Now is the time to act, and to act severely against the enemy -- there is a need to make a huge explosion," Ali told the gathering, some waving placards saying "Somalia is the graveyard of Crusaders".
"We are urging Muslims in Kenya to form their own liberation army and they will get our support," Ali added. "This is the time to show who you are."
Kenyan troops and tanks have crossed into Shabaab-held southern Somalia in an unprecedented military incursion to fight rebels it blames for attacking its territory and abducting foreigners from its soil.
Kenyan government spokesman Alfred Mutua said the country would "destroy al Shabaab's network within the shortest time possible," warning the militants posed a "clear and present danger" to the region.
"Kenya has no plans or intentions to stay in Somalia an hour beyond necessary," Mutua told reporters, but set no timeframe.
The threats come three days after two grenade blasts in Nairobi that killed one person and wounded dozens of others, and for which a Kenyan man has pleaded guilty of involvement.
Kenyan police chief Mathew Iteere said the Kenyan youth had joined Shabaab forces, and called for vigilance to prevent possible attacks.
"We have credible information there is quite a number of young people who have crossed into Somalia to fight for al Shabaab," Iteere said, noting that they came from several of Kenya's ethnic groups, not just ethnic Somalis.
"We are talking about the bad elements from any of the communities."
Four people died Thursday when gunmen fired a rocket-propelled grenade at a vehicle carrying government officials in Elwak region close to the Kenyan-Somali border.
"We suspect they are al Shabaab because they escaped towards Somalia after the incident," said regional administrator Joseph Ole Sereni.
Kenya launched its assault after attacks on its territory and the abduction of several foreigners it blames on Shabaab extremists, claims the rebels deny.
Troops and tanks have pushed some 100 kilometres (60 miles) into Shabaab-controlled southern Somalia, where fighting raged Thursday.
"Kenyan troops were attacked by the Shabaab in what is the first clash with the militia forces" on Somali soil, said Kenyan military spokesman Emmanuel Chirchir.
"Nine al Shabaab were killed with others escaping with injuries," he said, adding that two Kenyan soldiers were wounded, one critically.
The Shabaab for their part said they had killed 15 Kenyan soldiers.
"We have carried out the first ambush attack on the enemy of Allah this afternoon near Tabto village and the Mujahedeen fighters have killed fifteen of the enemy soldiers in that ambush attack," Sheikh Abdulaziz Abu-Musab, a Shabaab spokesman, said during a press conference in Mogadishu.
Kenyan troops have been bogged down by heavy rains and made slow progress towards the strategic Shabaab-held port of Kismayo.
Mutua said the assault had been planned for months in advance, adding the operation had been "going on for quite some time."
Kenya's assault prompted an angry response from Somali President Sharif Sheikh Ahmed, who on Monday warned he would "not allow forces entering its soil without prior agreement."
Sharif's comments perhaps echo worries of some Somalis who oppose the Shabaab, but who have expressed concern that Kenya's attack on the rebels may also include an attempt to carve out a buffer zone of control in the south.
However, Somali Prime Minister Abdiweli Mohamed Ali on Thursday said "Kenyan forces are inside Somali territory because of our official consent, the two countries have a bilateral agreement to fight Al-Shabaab."
Mutua also dismissed reports that Western nations were actively supporting Kenyan operations in the war-torn nation.
"With the United States, France, Britain and all our partners, we discuss matters with them," he said.
"But they are not involved at all in the current operations going on inside Somalia."
