By Ibrahim Mohamed and Feisal Omar
MOGADISHU (Reuters) - Al Qaeda-linked al Shabaab militants rushed reinforcements to Somalia's southern border with Kenya on Monday in response to a Kenyan cross-border offensive and threatened to take the "flames of war" across.
Kenyan troops launched an offensive with Somali forces in southern Somalia on Sunday in a risky attempt to secure the border with the anarchic country and end a wave of kidnappings that have hit Kenya's reputation among tourists and investors.
The Islamist militants, who have wrested control of much of Somalia from a weak internationally backed interim government and warlords, threatened major retaliation.
Residents in southern Somalia said both sides were heading toward the town of Afmadow, a rebel stronghold until air strikes on Sunday on nearby rebel camps dislodged their grip.
Afmadow town elder Abdi Gaboobe said the militants had deployed fighters around the city and were digging trenches in anticipation of a clash, while another resident said hundreds of fighters were seen hiding in jungles surrounding the town.
Sugule Farah, a telephone operator in Afmadow, said he saw al Shabaab fighters chanting 'Allahu Akbar (God is Great)."
"People are scared and are trying to find ways to move either to Kismayu or to refugee camps. But it's raining and no more cars can leave," Gaboobe said.
The Kenyan military said the operation, involving air and ground raids, was going well and had driven the rebels back.
As columns of al Shabaab technicals -- open-backed 4x4s mounted with heavy machine guns -- raced toward Afmadow, the militants' spokesman warned Kenya to pull its troops out of Somalia or face retribution.
"Kenyan troops have entered 100 km (62 miles) into Somalia and their planes bombarded many places and killed residents," al Shabaab spokesman Sheikh Ali Mohamud Rage told reporters south of Mogadishu. "We shall come into Kenya if you do not go back."
"Do not let the flames of this war spill over into your country," Rage said in a statement.
A Kenyan military officer based in Liboi, a border post on the frontier about 120 km (75 miles) to the west of Afmadow, said the joint military operation had driven al Shabaab from the border area.
"Our team inside Somalia is doing well. So far, we have uprooted al Shabaab from Dhobley after the air and ground raids," said the officer, referring to the Somali district across the border.
"We are working with other friendly armed groups ... to fight the common enemy, al Shabaab," he said.
Abdinasir Serar, a senior commander in the Ras Kamboni militia, allied to the Somali government, told Reuters he was seven km (five miles) from Afmadow.
"We are near Afmadow which we are waiting to capture in the coming hours. It's raining and the roads are very wet but our troops are very close to the city."
Earlier on Monday, a resident in the village of Qoqani, which lies 90 km (56 miles) inside Somalia between the border and Afmadow, confirmed Kenyan troops backed by heavily armored vehicles were preparing to move on.
In the southern port city of Kismayu, the nerve center of al Shabaab's southern operations, locals said the insurgents were heading north of the road to Afmadow to confront the advancing Kenyan and Somali troops.
"All al Shabaab's fighters and their armed vehicles in Kismayu have taken the road toward Afmadow," resident Ismail Aden said. "People are afraid here."
Kenya has blamed al Shabaab for the abduction of two Spanish aid workers last week from the world's biggest refugee camp close to the border with Somalia.
Rage dismissed the accusation and said the militants were also not responsible for the recent kidnapping of a disabled French woman and a British holiday maker from the north Kenyan coast in two separate incidents.
"Kenyan troops have entered Somalia under the pretext of chasing hostages. Al Shabaab is not behind any abduction."
Security sources say the British and French women are being held in al Shabaab controlled territory in central Somalia, highlighting cooperation between the militants and criminal networks such as pirates who hijack vessels for ransom.
East Africa's biggest economy has long looked nervously at its anarchic neighbor and its troops have made brief incursions into Somali territory in the past.
The latest operation appeared to be a significant escalation in military involvement but one which also risks dragging Kenya deeper into Somalia's two-decade civil war and drawing retaliatory attacks on Kenyan interests by al Shabaab.
Keen to avoid a spillover of violence by al Qaeda-trained foreign jihadists seeking haven in Somalia as well as al Shabaab rebels entrenched in the south, Nairobi has considered creating a buffer zone along its border.
Kenya has already trained thousands of newly recruited Somali soldiers to man the frontier. It also provides logistical and intelligence support to Somali government troops and government-friendly militia.
"The Kenyan military strategy looks intent on supporting these military groups," said Rashid Abdi, a Nairobi-based Somalia analyst at the International Crisis Group.
"I don't think Kenya has any intention of occupying southern Somalia. For one, they have no capacity to do that," he said. "What they probably want to do is step up the support of these militia groups so that the militia groups take territory."
A senior military official in Mogadishu said Western-backed government troops would be aiming to wrest control of southern Somalia from al Shabaab.
"I know TFG (government) forces and allied militia forces have been carrying out operations in that area even before Kenyan authorities arrived. I know they have been making some advances," the official told Reuters.
"For TFG forces their intention is to liberate the whole of southern Somalia," he said.
Horn of Africa experts say al Shabaab has resisted following through on previous threats to target Kenya, which diplomats say is an important money laundering destination and, in places, fertile recruitment zone for the rebels.
"There will now be a strong argument now for 'let's punish Kenya for this'," said ICG's Abdi.
(Additional reporting by Sahra Abdi and Abdi Sheikh in Mogadishu, Noor Ali in Isiolo, Richard Lough and Yara Bayoumy in Nairobi; writing by Richard Lough; editing by Yara Bayoumy and Philippa Fletcher)