Pakistan's leaders are bristling at a series of recent broadsides by U.S. officials accusing the South Asian nation's spy service of aiding the Taliban, setting the stage for what could be contentious meetings when top American officials visit Islamabad this week. The American allegations and the Pakistani response are, to a certain degree, an effort by both sides to stake out positions as they prepare to thrash out the details of the benchmarks to be set under President Barack Obama's new strategy for overcoming the Taliban and al Qaeda and stabilizing Afghanistan and Pakistan. The stakes for both sides are enormous. American officials say the war in Afghanistan will be decided in the Pakistani tribal borderlands where the Taliban and al Qaeda dominate. Islamic militants, meanwhile, are stepping up their attacks in Pakistan's heartland, prompting U.S. and Pakistani officials to warn that the country's very existence is at risk. Illustrating the threat, a senior Pakistani Taliban commander, Hakimullah Mehsud, warned Sunday that the militants would carry out two suicide attacks a week like one in Pakistan's capital Saturday unless the U.S. stops launching missile strikes against militant leaders in the mountains along Pakistan's border with Afghanistan. In Saturday's attack a suicide bomber penetrated a paramilitary police post in the heavily guarded heart of Islamabad, killing eight people just miles from the presidential palace. On Sunday, a suicide bomber killed at least 22 just 50 miles south of Islamabad after being stopped at the entrance to a crowded Shiite Muslim mosque in Chakwal. And in yet another attack Saturday, a suicide car bomber wounded three soldiers at the gate of an army base in the tribal areas. Those two attacks appeared to be unrelated to Mr. Mehsud's threat, made in a phone call to the Associated Press. The new U.S. policy includes billions of dollars in fresh military and development aid for Islamabad. Washington says it plans to closely monitor how the money is spent. Richard Holbrooke, the U.S. State Department's special representative on Afghanistan and Pakistan, and Adm. Mike Mullen, the chairman of the U.S. Joint Chiefs of Staff, are due in Islamabad this week for the first face-to-face talks between top officials from the two countries since Mr. Obama announced his plan late last month. With Pakistan frustrated at what it says are heavy-handed American efforts to dictate strategy for fighting the militants, officials on both sides are trying to head off a breakdown in relations that could scuttle the Obama administration's ambitious plans. Washington's renewed frustration with Islamabad has become increasingly clear amid a flurry of statements by American officials about the need for Pakistan's military and Inter-Services Intelligence spy agency to sever their remaining links to Islamic militant groups. They nurtured those groups to exert influence in Afghanistan and battle Indian forces in contested Kashmir. Pakistan insists its efforts to cut deals with some Taliban factions while taking on others can, over the long term, bring peace to the region. American officials say they are skeptical about that strategy, in large part because Islamabad has been hesitant to go after the Taliban factions attacking U.S. and North Atlantic Treaty Organization forces in Afghanistan, and instead has been focusing on militants sowing violence in Pakistan. U.S. officials attribute that reluctance partly to their claim that some elements of Pakistan's military and intelligence agencies still see the militants as useful assets and continue to aid them. Pakistani officials privately acknowledge maintaining contacts for intelligence-gathering purposes, but insist they provide no support to the militants. Meanwhile, a United Nations worker abducted more than two months ago headed home Sunday, a day after being released unharmed by a separatist group in the province of Baluchistan. John Solecki, an American who is a senior official with the United Nations' refugee agency, was discovered Saturday evening abandoned in a remote village. His captors -- the previously unknown Baluchistan Liberation United Front -- had threatened to behead Mr. Solecki.