Kenya's foreign minister has told the BBC his country has sent troops into neighbouring Somalia to target the Islamist insurgent group al-Shabab.Moses Masika Wetangula said Kenya was "defending itself", after a spate of suspected al-Shabab kidnappings.But a Somali diplomat at the UN told the BBC that if the reports were true it would be a violation of sovereignty.Several Westerners have been seized in Kenya by suspected Somali militants and reportedly taken into Somalia.Two Spanish aid workers were abducted from Kenya's sprawling Dadaab refugee camp on Thursday.A British woman and a French woman have been kidnapped from remote beach resorts over the past month, dealing a major blow to Kenya's tourism industry.Al-Shabab has denied involvement in those kidnappings.'Air strikes'Eyewitnesses in Somalia said columns of armoured Kenyan vehicles backed by tanks crossed into Somalia on Sunday.Mr Wetangula said Kenya was forced to act after a rash of cross-border kidnappings."If you are the Kenyan government or a Kenyan, what would you have done? Will you clap for [the kidnappers] and say they have done a good job?"No. You must defend your country, you must defend the security of your people and in doing so, you have to go for these people where they are."He said Kenya was acting at the request of the Somali government."What we are doing is in pursuit of a request by the government of Somalia and also our own interest as a country to fight a group that is terror-based, a group that is causing terror and mayhem and killings and kidnappings on both Somali nationals and Kenyan nationals and visitors to this region, and a group that is undesirable in our midst if we are to have peace in Kenya and Somalia."Witnesses said tanks and up to 40 Kenyan vehicles carrying soldiers had passed through the Somali town of Dhobley, near the border.Somalia has not yet confirmed that Kenyan troops were on its territory.The BBC's East Africa correspondent Will Ross says the Somali government is propped up by Ugandan and Burundian peacekeepers, so it is perhaps not surprising that there appears to be a reluctance to admit to yet more foreign troops.Senior Somali military commander Abdi Yusuf told Reuters news agency that warplanes had attacked two al-Shabab bases in southern Somalia but could not confirm if the jets were Kenyan."I can't identify the military aircraft, but our neighbour Kenya is fully supporting us militarily and our mission is to drive al-Shabab out of the region," he said.Senior al-Shabab figure Sheikh Hassan Turki vowed to repel the Kenyan forces."Kenya violated the territorial rights of Somalia by entering our holy land, but I assure you that they will return disappointed, God willing," he said."Mujahideen fighters will force them to test the pain of the bullets."Previous foreign interventions in Somalia have ended in embarrassing withdrawals - the US in 1992 and Ethiopia in 2006.Correspondents say many Kenyans will fear their country could be bogged down in a long, unwinnable conflict.Civilian fearsSomalia's UN envoy, Omar Jamal, said if confirmed, a military incursion by Kenya would be "a very serious territorial intrusion by a foreign country"."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."Our correspondent says some Kenyans fear their country could become a target for more al-Shabab attacks if it becomes more deeply embroiled in Somalia's conflict.The development comes days after the two Spanish aid workers with Medecins Sans Frontieres (MSF), named as Blanca Thiebaut and Montserrat Serra, were taken from Dadaab, the world's largest refugee camp.Just 80km (50 miles) from the Somali border, Dadaab currently houses nearly half a million refugees, most of whom are Somalis who have fled conflict and famine.A Kenyan driver working for the Care charity was abducted from Dadaab on 21 September.Last month, 56-year-old Briton Judith Tebbutt was kidnapped - and her husband David killed - by gunmen while the couple were on holiday in a remote Kenyan resort at Kiwayu.On 1 October, a 66-year-old French woman was seized by an armed gang on Kenya's northern resort island of Manda and taken to Somalia.The UK Foreign Office has advised against all but essential travel to the Kenyan coast near the Somali border.