LONDON - Four coalition soldiers - including two British Royal Marines - have been killed in separate attacks in Afghanistan in the last 24 hours, strengthening concerns that the Taliban is stepping up its offensive following the death of Osama Bin Laden. The two Royal Marines were killed in an explosion while on patrol on Friday. And NATO has now confirmed that two more soldiers were killed in attacks in the south of the country earlier Saturday. The latest fatalities take the death toll for Coalition forces to 13 this week. The two British soldiers, from 42 Commando Royal Marines, were killed by an improvised explosive device while on patrol in the Loy Mandeh area of Helmand Province. British Ministry of Defence spokesman said: 'The two Royal Marines were on patrol, disrupting insurgent activity when they were killed by an explosion caused by an IED. 'Our thoughts and prayers are with their families and friends in their time of grief.' The soldiers have not been named, although next of kin have been informed. Details of Saturday morning casualties have not been released, but a spokesman for the International Security Assistance Force said that one soldier was killed in a roadside bomb while another was killed in a suspected insurgent attack. On Thursday a powerful bomb exploded in southern Afghanistan, killing seven American troops. The soldiers were on foot patrol in a field in the Shobarak district of Kandahar province when the bomb went off. The Taliban claimed responsibility for the blast, which also killed two Afghan policemen near the Pakistan border in an area that was thought to be free of terror. An eighth NATO serviceman died on Thursday in a helicopter crash. And on Monday, another British soldier, Colour Sergeant Kevin Fortuna, died when a Taliban bomb ignited as he led his men out on patrol in the Saidabad Kalay area of the Nahr-e Saraj district of Helmand. Fundamentalist groups promised retaliation for the death of Bin Laden, who was killed when US, Navy Seals stormed his compound in Abbottabad, Pakistan on May 2. When Bin Laden died, insurgents announced the start of a spring offensive against NATO forces and Afghan authorities, which have been bracing themselves for a series of spectacular and complex attacks. General David Petraeus, the top US and NATO commander in Afghanistan, has said the size of the withdrawal will depend on conditions on the ground. The alliance has committed itself to handing over control of security in the country to Afghans by 2014.Forty-four international service personnel have been killed in Afghanistan so far this month, bringing the death toll for the year to 195.