Islamabad has consistently rejected the accusations.
Speaking in Kabul today, Alexander also called on Pakistan to drop its plan to mine parts of the border to prevent militants from crossing into Afghanistan.
Alexander said land mines represent a "very serious threat" to the security of local populations.
Afghan President Hamid Karzai has also rejected the plan.
The UN representative also expressed his concern about support in Pakistan for the Taliban movement and urged Islamabad to take more action against Taliban leaders.
(compiled from agency reports)
Afghan President Hamid Karzai (left) with Pakistani President Pervez Musharraf in Islamabad in October 2005 (epa)
ACROSS A DIFFICULT BORDER. The contested border between Pakistan and Afghanistan is some 2,500 kilometers long and runs through some of the most rugged, inhospitable territory on Earth. Controlling that border and preventing Taliban militants from using Pakistan as a staging ground for attacks in Afghanistan is an essential part of the U.S.-led international coalition's strategy for stabilizing Afghanistan. Officials in Kabul have been pointing their fingers at Pakistan for some time, accusing Islamabad or intelligence services of turning a blind eye to cross-border terrorism targeting the Afghan central government. Many observers remain convinced that much of the former Taliban regime's leadership -- along with leaders of Al-Qaeda -- are operating in the lawless Afghan-Pakistani border region.... (more)