A U.S.-led coalition soldier and seven Afghan civilians were killed in the latest violence in the country's south, which has been hardest hit by the surge in insurgent attacks.
Speaking to a group of tribal elders from eastern Afghanistan, Karzai said he did not want to form militias that could clash with rival tribes.
"We just want to strengthen the districts to safeguard them from terrorist attack," he said.
Although they would not speak for attribution because of the sensitivity of the topic, Western diplomats briefed on the plan said they worried it could fuel factional fighting by giving weapons to forces loyal to warlords with long histories of factional disputes.
Karim Rahimi, Karzai's spokesman, said that would not happen because, the government believes, the recruits will be loyal to Kabul. He described the new recruits as "community police."
"They are to strengthen the security setup in Afghanistan," he said.
The president did not say how many tribal fighters would be recruited. But he said there would be a dramatic increase in the ranks of security forces in some areas.
He told the elders that in one troubled district in southern Kandahar province that there were only 45 police for a population of 65,000.
"We need about 150 police in that district for it to be strong, so we need to build the force from within the community," he said.
Defense Minister Abdul Rahim Wardak told reporters that the tribal forces would "take their command from each district police chief."
He said local security forces would also be given better weapons and bulletproof vests.
The surge in fighting has killed more than 500 people, mostly militants, since mid-May and raised fears of a Taliban resurgence. Wardak said the rebels have stepped up attacks to scare NATO countries from deploying troops there.
He said that violence had increased in the south because the Taliban was making an all-out push to scare Britain, Canada, the Netherlands and Romania from deploying some 6,000 troops to the region.
The soldiers are scheduled to resume responsibility for the region from the U.S.-led coalition next month.
"They just want to take advantage of this period of transition from coalition to NATO and they want to have maximum impact," Wardak said of the insurgents.
He said that once NATO deploys extra troops to the region — effectively doubling the number of foreign combat forces — and the government deploys more soldiers there, "I am absolutely sure that the situation will improve drastically."
In the latest violence, a U.S.-led coalition soldier was killed when a bomb hit his armored vehicle during a search of a village in Ghazni province, the U.S. military said in a statement.
U.S. military spokesman Sgt. Chris Miller said he was not permitted to disclose the soldier's nationality.
Also in Ghazni, unidentified gunmen killed three Afghans late Saturday as they were driving near the provincial capital, said Ali Ahmad, a local police commander.
In Kandahar province, gunmen killed four Afghan laborers working for an Indian road construction company, said Daod Ahmadi, a government spokesman. The men were driving home with $8,000, which was stolen from them. It was not immediately clear whether the Taliban or thieves were responsible.