Four service members from the NATO-led International Security Assistance Force (ISAF) were killed by a roadside bomb in eastern Afghanistan on Saturday, the force said, continuing a trend of rising violence in recent weeks. ISAF gave no other details about the latest incident. Most of the ISAF troops serving in the hotly contested east, where a fragmented insurgency is being fought near the border with Pakistan, are Americans. At least 220 foreign troops have been killed in Afghanistan in 2011, 57 in May when the Taliban began a spring offensive, making it the deadliest month of the year for foreign troops. The Taliban vowed to attack foreign military targets, as well as the Afghan government and security forces, when they launched their offensive at the beginning of the traditional warm-weather fighting season in Afghanistan.