As 30,000 U.S. troops begin to deploy to Afghanistan, fears are rising in Pakistan that a stepped-up war just over the border could worsen the increasingly bloody struggle with militancy here. Residents in border areas such as the violence-plagued city of Peshawar worry that a tide of militants could flee Afghanistan to seek targets in Pakistan. Doubts linger among Pakistani security officials about the Americans' ability to intensify the campaign against the Taliban without further destabilizing Pakistan's vast southwestern border or the already volatile tribal areas in the northwest. 'With a surge in American troops across the border, the militants facing pressure could come to our place, which will destroy peace and stability,' said Haji Adam Khan, the top official in Qilla Abdullah, a mountainous Pakistani district that abuts Kandahar province in southern Afghanistan. U.S. officials disagree with that assessment, reflecting the undercurrents of mistrust between allies whose relationship President Barack Obama has called crucial to success in Afghanistan. Pakistani reservations about the U.S. strategy highlight the limits of an American-led war campaign in Afghanistan that must stop at a famously porous border _ Pakistan does not allow U.S. troops to fight on its soil _ even if the enemy crosses over. A wave of bombings has swept Pakistan since October, devastating Peshawar but also reaching far beyond the troubled northwest. Attacks on places believed to be safe, such as the military headquarters in Rawalpindi and a popular market in the eastern city of Lahore, have struck fear into the population. Last week, Pakistan's foreign minister warned in a statement that the U.S. troop buildup could magnify the problems by bringing an 'influx of militants and refugees from Afghanistan into Pakistan.' The U.S. invasion of Afghanistan in 2001 led thousands of Taliban and al-Qaida members to flee the fighting and seek refuge in Afghanistan. Since then, insurgents have continued to use Pakistan as a staging ground for launching attacks into Afghanistan and within Pakistan. When U.S. troops launched a major offensive in Afghanistan's Helmand province last summer, Pakistani officials feared a fresh influx of militants into their territory. But that did not occur, U.S. and Pakistani military officials said. 'I think it's somewhat exaggerated,' Talat Masood, a retired Pakistani general and security analyst, said of Pakistani fears about a surge of militants. 'Why should they come here? They already have all the space they need in Helmand' and other Afghan border provinces, he said. U.S. military officials said they are working closely with Pakistan's security forces ahead of the troop increase and note that a stream of top American officials has visited Pakistan to seek support for the strategy. A senior U.S. military official in Pakistan said a spillover of militancy or refugees is unlikely, in part because of regular meetings in which officials from both nations discuss 'where we think the forces are going to go, and if that would cause issues.' The Pakistani military is less sure. It is already stretched thin by military operations in South Waziristan and other tribal areas against the Pakistani Taliban, the group it blames for most domestic attacks. The American plan to focus on control of Afghan cities and improve governance is positive, a Pakistani military official said, but Pakistan fears that U.S. forces might also 'go on a wild goose chase' of targeting Taliban fighters without guarding borders. That could push fighters into Pakistan's tribal areas, reversing military gains there, or into Baluchistan in the southwest, where Pakistan _ which concentrates its forces in the northwest and along the eastern border region with arch-rival India _ has insufficient troops, the official said. Whether or not the new U.S. strategy in Afghanistan fuels violence in Pakistan, the border is likely to be a vexing problem for troops. It is certain that militants will seek refuge in Pakistan, but it is just as likely that extremists based in Pakistan will 'follow the sound of the guns' into Afghanistan, a senior U.S. military official in Afghanistan said. Bolstering border security is a 'key part of the strategy' to prevent both militant movement and the import of bomb-making supplies into Afghanistan, the U.S. official said. But the official also suggested that Pakistan could assist more, by moving troops from the Indian border and by focusing more on 'eliminating' key insurgent leaders to consolidate its military gains. Pakistan has not captured top Pakistani Taliban insurgents during its two-month- old operation in South Waziristan and has declined to pursue the Haqqani network and other groups that attack U.S. forces in Afghanistan. 