WASHINGTON (AFP) - US Secretary of State Hillary Clinton described relations between the United States and Pakistan on Sunday as 'very challenging, a tacit admission of strained ties between the key allies. Speaking after Pakistan boycotted a trilateral meeting on Afghanistan in Brussels in protest at a drone attack that killed 39 people, Hillary said Islamabad was caught between wanting to help and dealing with its own extremist threat. Its a very challenging relationship, because there have been some problems, she said in an interview with the ABC programme 'This Week. Its a very difficult relationship, because Pakistan is in a hard position, trying to figure out how its going to contend with its own internal extremist threat, she said. But I think, on the other hand, weve also developed good lines of communication, good opportunities for cooperation, but its something we have to work on every day. Hillary noted the cooperation of the Pakistani government in securing the release of Raymond Davis, a CIA contractor accused in a double murder, as well as in the fight against Al-Qaeda and extremism. We were very appreciative of getting our diplomat out of Pakistan, and that took cooperation by the government of Pakistan, she said. We have cooperated very closely together in going after terrorists who pose a threat to both us and to the Pakistanis themselves. Washington always insisted that Davis had diplomatic immunity from prosecution. He was released earlier this month after a payment of $2 million, but Hillary Clinton has denied the United States paid any compensation.