An injured blast victim arrives in an ambulance at Kenyatta National Hospital in Nairobi on March 31, 2014. Picture: Noor Khamis

Somalia's al-Qaeda linked al-Shabaab vowed on Thursday to move their war to neighbouring Kenya, in a broadcast by a top commander urging fighters to rise up against Nairobi.

Kenyan troops are battling the hardline Islamists inside Somalia and the insurgents have already launched scores of bombings and killings in revenge, but the al-Shabaab threat comes amid fresh security warnings of the risk of attacks in Kenya.

“The war will be shifting to Kenya, if they kill a Somali girl we kill a Kenyan girl,” Fuad Mohamed Khalaf, one of the al-Shabaab's most senior commanders, said in a radio broadcast.

“We are urging all the Muslims in Kenya... to fight the government of Kenya inside that country, because Kenyans killed your people including children,” Khalaf said in a speech broadcast on the al-Shabaab's Radio Andalus.

Kenyan troops crossed into southern Somalia in 2011 to fight the al-Shabaab, later joining the now 22 000-strong UN-mandated African Union force battling the Islamists.

Fighter jets, believed to be from Kenya, pounded al-Shabaab strongholds this week, as part of the latest push against the insurgents by the AU force, which said it was targeting its “senior leadership and foreign fighters” and boasted of having killed over 50 rebels.

“When their soldiers and war planes kill your people, God permits you to retaliate accordingly, we will fight the Kenyans,” Khalaf said, viewed as second in importance only to al-Shabaab chief Ahmed Abdi Godane.

“We shall kill those inside our country and make their graveyards here in Somalia - and we will kill them in their homes as well,” added Khalaf, for whom the United States has offered a $5 million bounty.

The al-Shabaab, who claimed responsibility for the September 2013 attack on Nairobi's Westgate mall in which at least 67 people were killed, have also been blamed for a string of grenade blasts and killings.

Last week a double bomb attack in a Nairobi market left 10 people dead and scores wounded, while the United States has said it was preparing to cut staff levels in Kenya because of the mounting threat of attacks.

Hundreds of British tourists were also evacuated from beach resorts near Kenya's port city of Mombasa following new warnings of terror attacks from Britain's Foreign Office.

The al-Shabaab said its guerrillas were behind a deadly ambush on an army convoy in Kenya's northeastern Mandera region on Monday in which at least eight people were killed, close to the border with Somalia.

Khalaf said the al-Shabaab had trained fighters and vowed more would be sent to carry out attacks inside Kenya.

“We have trained the people... they are the ones who carried out the Mandera attack,” he said. “More are going to be sent soon.”

Khalaf, who the US says is both a al-Shabaab military commander and key fundraiser holds both Somali and Swedish nationality, and reportedly spent over a decade in the Swedish capital Stockholm.

Hardline al-Shabaab insurgents once controlled most of southern and central Somalia.

After withdrawing from fixed positions in the capital Mogadishu nearly three years ago, they have lost most large towns to the AU and government soldiers. However, they still regularly launch guerrilla raids.

Kenya last month launched a mass round-up and deportation of ethnic Somalis as part of a counter-terrorism crackdown, criticised by rights groups but which police say is key to weeding out al-Shabaab sympathisers.

Threats by the al-Shabaab have badly dented tourism.

The number of foreign visitors to the country - a top safari and beach destination - slumped by 11 percent in 2013, when the country was gripped by fears of election-related political violence.

The current year is expected to see an even greater drop, particularly in the wake of the high-profile attack on the Westgate mall. - AFP