Andrew Buncombe and Omar Waraich: Arrival of more troops will arouse suspicion in Pakistan

While it is to Afghanistan that additional American troops are being dispatched, Barack Obama's administration has made it clear that its policy is dependent on "getting it right" in neighbouring Pakistan – a country that has very different regional priorities to the US.

In public, the US is full of praise for Islamabad, yet in private it continues to demand far greater efforts to take on Pakistan-based militants responsible for carrying out cross-border attacks on Western troops.

Until now, Pakistan has focused on targeting militants who pose a direct threat to the country from within. But recently, Mr Obama dispatched a letter to Pakistan's President, Asif Ali Zardari, urging him to expand anti-militant operations and rally his nation behind a more determined campaign.

Most notable among those militants who use Pakistan as a staging ground is Mullah Omar, suspected by Western intelligence agencies to be based around the city of Quetta, and the Haqqani network based in North Waziristan. Critics allege that the Pakistan army sees these militants as potential proxies who could be used for leverage in Afghanistan. In his letter, Mr Obama said Pakistan must put an end to any such "ambiguity".

Pakistan's concerns over the new US policy are plentiful. While Islamabad might hope that the presence of additional US troops could push the Taliban towards the negotiating table where it could then play a crucial role, it is also concerned that a significant surge may spark a spillover of militants into the already troubled south-western province of Baluchistan. There are also worries that a troop build-up could complicate the military offensive in South Waziristan, the Pakistani Taliban's main stronghold along the border with Afghanistan.

There is also anxiety about any indication of the eventual withdrawal of US troops, particularly if Pakistan's own interests in Afghanistan – the establishment of a pro-Islamabad government, enhanced Pashtun power and diminished Indian influence – are not met. Analysts say talk of a US withdrawal would likely diminish Pakistan's already lacklustre appetite for targeting those militants Washington wants it to confront.

"They have always felt that the United States would run away and they would be left with the mess – just like they were in the 1990s," Bruce Riedel, a former CIA analyst who recently worked on the Obama administration's review of Afghanistan and Pakistan policy, told Reuters. "It's very hard to dispel that image."

Those more critical of Pakistan believe it will do well whatever the outcome of the surge. "If we stay the course, they make extra money from providing supply routes into Afghanistan and becoming more 'indispensable'," said Christine Fair of Washington's Georgetown University. "If we give up and pull out, then it's vindication of their continuing support for the Taliban."

Despite the US's offer of additional civilian and military aid and what Secretary of State Hillary Clinton told Pakistan would be a closer relationship when she visited in October, there is no doubt that Washington's additional demands have not been well-received in Islamabad.