WASHINGTON: In a tough message to Pakistan James Mattis , President-elect Donald Trump 's pick for US defense secretary, has said that if confirmed, he would tell Islamabad the need to "expel or neutralise" externally-focused militant groups operating with impunity within the country.In his written written submission to the Senate Armed Services Committee ahead of his confirmation hearing yesterday, Mattis noted that "conditioning our security assistance" to Pakistan has a mixed history, "but I will review all option.""If confirmed, I will work with the State Department and the Congress to incentivise Pakistan's co-operation on issues critical to our national interests and the region's security, with focus on Pakistan's need to expel or neutralise externally-focused militant groups that operate within its borders," said Mattis.The 66-year-old Marine general, who retired in 2013 after serving as commander of US Central Command, was responding to a question if he supports conditioning US aid to Pakistan.Nick-named as "Mad Dog" Mattis, the retired general told lawmakers that Pakistan has "learned some hard lessons" because of its dealings with the Afghan Taliban, as violence in that country reflects."I believe they should do more to collaborate with their neighbour. We should urge Pakistan to take further actions against the Taliban and the Haqqani network," Mattis said.Haqqani network, which is linked to al-Qaeda, has been blamed for several deadly attacks against Western and Indian interests in Afghanistan, including the 2008 bombing of the Indian mission in Kabul."Sanctuaries and freedom of movement for the Afghan Taliban and associated militant networks inside Pakistani territory is a key operational issue faced by the Afghan security forces. If confirmed, I will examine efforts to deny sanctuary to the extremist forces undermining the stability and security of Afghanistan," he said, responding to a query.Arguing that countries in the region "have the responsibility to support the reconciliation process" in Afghanistan, Mattis said the countries in the region "should increase pressure on the Afghan Taliban and associated militant networks" to stop their campaigns of violence.He said US' ties with Pakistan have had highs and lows."We have long faced a lack of trust within the Pakistani military and government about our goals in the region, If confirmed, I will work to build the trust that we need for an effective partnership," he said.For years Pakistan has battled internally-focused extremist organisations within its border and with US help, he said."In a sign of its commitment, its military has suffered significant casualties in this counterinsurgency effort," Mattis said, adding that the US has conducted military exercises with Pakistan in an effort to increase thrust and interoperability.