Two high-ranking Reagan Administration officials will hold talks in Pakistan on a range of issues including Afghanistan, a State Department spokesman said. Under Secretary of State Michael H. Armacost and Robert Oakley, Middle East director of the National Security Council staff, left Saturday for Pakistan, the spokesman added. Armacost has asked to see senior Afghan resistance leaders, the Washington Post reported, adding that President Reagan sent along a message reassuring the rebel leaders of continued U.S. support. Meanwhile, the Afghan government said a fifth relief convoy reached the beleaguered town of Khost. An independent witness confirmed that some supplies were getting through, although the rebels denied that their three-month siege of the town has been broken.