Kabul ? A roadside bomb killed four American troops in eastern Afghanistan on Monday, driving the July death toll for U.S. forces to the highest monthly level of the war.

The latest deaths brought to at least 30 the number of American service members 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 U.S. since the 2001 U.S.-led invasion drove the Taliban from power.

July’s death toll for the entire U.S.-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.

U.S. commanders had predicted a bloody summer after President Barack Obama ordered 21,000 additional U.S. troops to Afghanistan in a bid to turn the tide against a resurgent Taliban and shift the focus on the global war against Islamic extremism from Iraq.

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 U.S.-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.