BERLIN (AFP) - Secretary of State Hillary Clinton on Thursday backed away from a US timeline to start pulling troops from Afghanistan in July, warning 'political expediency would benefit the Taliban. Speaking at a Nato foreign ministers meeting in Berlin, Hillary also warned of a 'violent spring fighting season in Afghanistan as the Taliban try to exert themselves in areas where Afghan forces are due to assume control. "We have to steel ourselves and our publics for the possibility that the Taliban will resort to the most destructive and sensational attacks we have seen," she said. Hillary lauded the 'heroic sacrifices by nations in the International Security Assistance Force in Afghanistan, and insisted there has been progress in fighting the insurgents. "We need to ensure that these sacrifices are not overtaken by political expediency and short-term thinking," Hillary told the Nato meeting in Berlin. "We need to worry less about how fast we can leave and more about how we can help the Afghan people build on the gains of the past 15 months," she said. Hillary said that Afghanistan was seeing real, but reversible, progress and pointed to President Hamid Karzai's March announcement that Afghan forces would assume security in key areas including the southern province of Helmand. "For the transition to be sustainable and irreversible, and for reconciliation and diplomacy to bear fruit, we must sustain our efforts," she said, according to her prepared remarks. "We need to underscore that we are transitioning, not leaving," she said. Despite public opposition, Obama's Republican political foes have attacked him over the July deadline, saying it would the wrong signal to the Taliban and discourage neighbouring Pakistan from acting against insurgents. The Obama administration has gradually de-emphasised the timeframe, instead saying that most US forces would leave in 2014 - the date set by last year's Nato summit for putting Afghans in charge of their own country's security. Hillary appeared to de-emphasise that date as well, saying that the United States was committed to "building an enduring partnership with Afghanistan that will last well beyond 2014." "The Taliban need to know that they cannot wait us out," she added. Support for the Afghanistan war, launched in pursuit of Al-Qaeda after the September 11, 2001 attacks in the United States, has also dwindled among US allies. But Clinton renewed a US call for NATO nations to drum up $1 billion to help sustain the Afghan National Army. While keeping up the military effort, the Obama administration has also embraced efforts for reconciliation in Afghanistan, concluding that there is ultimately no political solution to the conflict.