A roadside bomb killed four American soldiers in eastern Afghanistan on Monday, driving the July death toll for US forces to the highest monthly level of the war. The latest deaths brought to at least 30 the number of American troops who have died in Afghanistan this month - two more than the figure for all of June 2008, which had been the deadliest month for the US since the 2001 US-led invasion drove the Taliban from power. July's death toll for the entire US-led coalition, which includes American, British, Canadian and other forces, stands at 55 - well over the previous record of 46 deaths suffered in June and August of 2008. NATO's outgoing Secretary-General Jaap de Hoop Scheffer said Monday that terrorism would spread through the world if NATO forces fail in Afghanistan. "Al-Qaida would have a free run again, and their terrorist ambitions are global," he said in a speech at London's Chatham House think tank. "Those who argue otherwise - who say we can defend against terrorism from home - are simply burying their heads in the sand." Defense Secretary Robert Gates has warned that US-led forces must demonstrate progress in Afghanistan by next summer or face a public perception that the conflict cannot be won. Heavy losses this month have already triggered a public debate in Britain that the war in Afghanistan may not be worth the price. With more troops in the country, American and British forces have been striking deeper into Taliban strongholds in the south, hoping to establish enough security for Afghans to choose a president next month and cut insurgent supply lines into Pakistan. British military authorities said Monday that bombing attacks in southern Afghanistan soared nearly 43 percent for the first five months of this year over the same period last year. US troops have also upped efforts in eastern Afghanistan to curb the movement of terrorists to and from safe havens in Pakistan's tribal region. A NATO statement said the four soldiers were killed by an improvised explosive device in the east of the country but gave no further details. A US spokesman, Lt. Robert Carr, confirmed all four were Americans. It was unclear whether the blast occurred near the area of eastern Afghanistan where Pfc. Bowe R. Bergdahl, 23, was taken captive June 30. Bergdahl appeared on a Taliban video posted on the Internet over the weekend - a move denounced by the US command as a violation of international law. Also Monday, the British Ministry of Defense announced that a British soldier was killed the day before by a roadside bomb during a foot patrol in Helmand province. Roadside explosives now account for more than two-thirds of all casualties among the international force as the Taliban demonstrate greater skill in manufacturing and planting the bombs. Bombings rose by 25 percent in the first four months of 2009 over the same period last year, and the US command expects them to increase 50 percent this year to 5,700 - up from 3,800 last year.