WASHINGTON - US President Barack Obama on Wednesday held a video conference with Afghan President Hamid Karzai focusing partly on an unprecedented wave of attacks on Nato troops by their local comrades.The White House meanwhile rejected calls to slow US troop withdrawals in a “strategic pause” to assess the impact of the killings, which number 51 so far this year and have put the Nato effort to build Afghan forces under scrutiny. The two presidents dealt with a range of strategic issues, including efforts to stem insider attacks on US, coalition, and Afghan forces, the White House said in a statement. They also discussed the need for restraint in the face of “inflammatory materials,” an apparent reference to a film made on US soil deemed offensive to Islam which sparked violence in Muslim countries.Obama and Karzai, who spoke by telephone last week, also used their secure videoconference to assess implementation of a Strategic Partnership Agreement that is designed to see NATO forces hand over security of the country by 2014.Obama also expressed condolences on the eve of the first anniversary of the assassination of former Afghan president Burhanuddin Rabbani, an envoy to peace talks with the Taliban, by a suicide bomber, the statement said.The president’s spokesman Jay Carney meanwhile rejected a call by three top senators for a halt to the withdrawal of US troops from Afghanistan following the so called “green on blue” attacks on Nato forces.Republicans John McCain and Lindsey Graham, and independent senator Joe Lieberman, called for a “strategic pause” to assess the damage of attacks by renegade Afghans, warning “we cannot afford to rush to failure in Afghanistan.”“Over the past three years, the Administration has repeatedly deployed fewer forces than our commanders recommended, and is now drawing down those forces in larger numbers and at a faster pace than our commanders advised,” the senators said in a statement.The senators said Obama “has said that the drawdown of US forces from Afghanistan would be responsive to conditions on the ground.“We believe those conditions are now worrisome enough to justify an immediate suspension of further US troop withdrawals at this time,” the senators said.Carney said the Obama administration disagreed.“The president believes that it is absolutely essential to continue with the transition to Afghan security lead, that after a decade of war — more, in Afghanistan, it is time to wind down that war and to gradually transfer security responsibility to the Afghans,” he said.Carney said the attacks on Nato troops by Afghan soldiers were “a very concerning problem.”“Action is being taken to protect against those kinds of attacks, but it does not change the mission,” he said.Nato said Tuesday it was scaling back joint operations with Afghan forces after an unprecedented number of Western soldiers were shot dead by their local colleagues.All 33,000 extra troops sent to Afghanistan as part of the president’s surge strategy announced in December 2009 are due to be home by the end of this month.Decisions on the pace of future withdrawals in line with the 2014 deadline to hand over control of all of the country to Afghan forces are pending.The Taliban, the main militant group leading the insurgency, said Wednesday it had “forced” Nato commanders into the decision by sowing distrust among Afghan and foreign troops.According to Nato, the Taliban are only involved in a quarter of Afghan security personnel attacks on Western soldiers. It attributes the rest to grudges, misunderstandings and cultural differences.