"Up until now, they have sadly refused to back efforts for negotiations with the Taliban," Karzai told Der Spiegel weekly in comments reported in German and due to be published on Monday.
The Bonn meeting will seek to chart a course for Afghanistan after the NATO withdrawal, but a boycott by Pakistan has dealt a stinging blow to hopes for a roadmap.
Pakistan is seen as vital to any prospect of stability in the war-ravaged country a decade after US-led forces ousted the Taliban, which had offered safe harbour to al-Qaeda leader Osama bin Laden.
But Islamabad pulled out after the killing of 24 soldiers in NATO air strikes on two Pakistani posts a week ago, although sources close to the German foreign ministry said it would be kept informed of progress at the conference.
Karzai also appealed for continued aid to his war-ravaged nation after 2014 -- when NATO troops are due to pull out.
Stressing that Afghanistan will be "more than ever on the frontline," he said: "If we fail in this war, which threatens all of us, it will mean a return to the situation before 9/11."
The Afghan leader conceded that "sadly we have not been able to provide security and stability to all Afghans, this is our greatest failure."
Afghan foreign minister Zalmai Rasoul appealed on Saturday for international support for his country after NATO troops pull out.
"After 2014, we will continue to need long-term support from our friends in the international community," Rasoul said at a discussion forum in Bonn.
His German counterpart Guido Westerwelle vowed at the forum that the world would not abandon Afghanistan, while also stressing the importance of the role of women in the county, where they currently face major discrimination.
In an interview to appear in Sunday's Frankfurter Allgemeine Sonntagszeitung, Westerwelle again voiced his regret over the Pakistani boycott of the conference, which will gather delegates from 100 nations.
"Pakistan has more to gain from a stable and peaceful Afghanistan than any of its neighbours," he said.
In Bonn on Saturday, several thousand people -- 4,500 according to organisers -- demonstrated in protest at the conference and the German army's role in Afghanistan.