The Islamist militant group al-Shabab has attacked a naval base used by Kenyan and US forces in the popular Kenyan coastal region of Lamu.Witnesses reported hearing gunfire and seeing plumes of black smoke emerge from Camp Simba in the early hours of Sunday.Kenya's military said troops had driven out the insurgents from the base.Al-Shabab is linked to al-Qaeda and has its headquarters in neighbouring Somalia.The group has carried out a spate of attacks in the region since it was formed more than a decade ago.On 28 December, about 80 people were killed in a bombing in Somalia's capital, Mogadishu.The Kenyan Defence Forces (KDF) said "an attempt was made to breach security at Manda Air Strip" but the attack was repulsed.Four militants were killed, it added.A fire caused by the attack had been extinguished and the airstrip was now safe, a KDF spokesman said.Al-Shabab said it had "successfully stormed the heavily fortified military base" before taking "effective control of a part of the base".The group said the Kenyan military used warplanes to repel the attack.The Associated Press news agency reported that two aircraft, two US helicopters and multiple vehicles were destroyed at the airstrip.Al-Shabab said there were "severe casualties on both American and Kenyan troops stationed there", and seven aircraft had been destroyed.A journalist with Voice of Africa tweeted photos of what the militants said was a US aircraft that had been targeted.