Pakistan Friday welcomed the appointment of US ambassador Richard Holbrooke as special envoy to Afghanistan and Pakistan as an opportunity to improve stability in the troubled region. "Pakistan welcomes the appointment of ambassador Holbrooke and looks forward to enhanced and fruitful engagement with the special envoy to further the cause of peace and stability of the region," the foreign ministry spokesman said. US Secretary of State Hillary Clinton called President Asif Ali Zardari Thursday to inform him of the appointment, said the spokesman, Mohammad Sadiq. US President Barack Obama said Thursday Islamist extremists in Pakistan and Afghanistan posed a grave threat which his new administration would tackle as a single problem under a wider strategy.