Karzai said the Taliban's insistence on a withdrawal of Western troops before any talks was "not a meaningful gesture".
"The international community is here for success in defeat of terrorism, success in the defeat of extremism," Karzai told a news conference.
"Therefore, they have to be satisfied that they have achieved their objective before they can leave."
'Peace and security'
The US is sending an additional 30,000 troops to Afghanistan this year, while other nations are set to contribute 7,000 extra soldiers, to try to turn the tide in the war with the Taliban and al-Qaeda.
Washington has said that it then wants to begin drawing down its forces in mid-2011.
At the London conference, Karzai said that Kabul and its international backers would concentrate of wooing his "disenchanted brothers" who were fighting for money rather than ideology.
He said that he was been hoping that the leaders of anti-government groups would be removed from international "terrorist" blacklists to pave the way for talks.
The United Nations removed five names last week, but none of them were senior Taliban figures.
"We as Afghans are trying our best to reach as high as possible to bring peace and security to Afghanistan, but it has an international aspect as well. It is a bit more complicated," he said.
Karzai said he would convene the jirga in less than six weeks, before another international conference he intends to host in Kabul, the capital, some time in the next few months.
The Afghan president has consistently made overtures to the Taliban, and the West has been increasingly supportive of proposals to lure fighters back into the political process in a bid to end years of fighting in a war now into its ninth year.