Earlier this month, the government said a number of Hezb-i-Islami fighters, who lead an insurgency separate from the Taliban, had agreed to back the authorities after clashing with Taliban fighters over control of villages in the north.
David Chater, Al Jazeera's correspondent in Kabul, said the talks were a breakthrough for Karzai's efforts at reintegration and reconciliation of the fighters against his government.
"This is a direct result of negotiations with the Afghan government that took place in the Maldives in January," Chater said.
"It means they will perhaps attend the peace talks being called for the end of April. It adds a lot of momentum to Karzai's drive to reintegrate the insurgents [into mainstream politics]."
However, Chater warned that the negotiations had a long way to go.
"We don't have details of the peace plan yet, however, I've heard one of the demands is that all foreign troops should withdraw from Afghanistan by this July. That is impossible, which shows there is a lot of negotiation to go on yet."
Karzai has reached out to Hezb-i-Islami indirectly in the past and he included a former member in his latest cabinet reshuffle, but the delegation this week appears to be one of the most senior to participate in direct talks, Reuters said.
Hezb-i-Islami is one of the three groups that Nato forces recognise as the main insurgent factions, led by Hekmatyar, a veteran anti-Soviet guerrilla commander, civil war faction leader and former prime minister.
They have long fought Nato and Afghan government forces in the east and in pockets in the north.
The group has in the past claimed to share some aims with the Taliban, but has remained separate.
In recent months the Taliban has made some gains in areas controlled by Hezb-i-Islami.
Karzai has consistently made overtures to the Taliban to end their armed struggle, 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.