ISLAMABAD – Pakistan on Thursday expressed concern over the "increased Indian role" in Afghanistan and said that Islamabad would remain neutral in the Afghan affairs. "Pakistan has serious concerns about India using Afghan territory to destabilise Pakistan," Foreign Office spokesman Abdul Basit said at weekly press briefing. Abdul Basit said Pakistan is ready to play a constructive and positive role in Afghanistan but it is for Afghanistan to determine such a role. Pakistani forces have arrested 35 suspects in their investigation into a suicide bombing that killed three American soldiers and four Pakistanis on Wednesday in the country's northwest. Thursday's arrests were made by Pakistani police and members of the paramilitary Frontier Corps, a faction the US soldiers – part of a special operations team – were in Pakistan to train in counterinsurgency techniques. All the suspects are locals, a duty officer in the town of Balambat, where the bombing occurred, told The New York Times. "We are questioning them in an effort to trace those who orchestrated the suicide attack," a police official, Naeem Khan, told reporters. Investigators now believe Wednesday's explosion was caused by a car bomber rather than a roadside bomb with a remote detonator, he said. Meanwhile, thousands of NATO and Afghan troops are gearing up to launch a massive offensive in southern Afghanistan – the largest the country has seen since the 2001 US-led invasion. Announcement of the joint operation, meant to target Taliban strongholds in Helmand province, comes as NATO defence ministers gathered in Istanbul on Thursday to discuss the situation in Afghanistan. Analysts said that the target of the operation was believed to be the Marjah area, a stronghold of around 1,000 Taliban fighters. Marjah is also thought to be the hub of the Taliban-controlled opium trade – which provides them with most of their funding.