Khaliq Ahmad, an Afghan presidential spokesman, said: "Rather than beating about the bush, we must confront terrorists in a real manner.
"Fencing or mining the border is neither helpful nor practical. We're against it. The border is not where the problem lies."
He did not suggest an alternative to stop people from crossing the mountainous border to fight Nato forces in Afghanistan.
Pakistani rationale
The Pakistan foreign secretary said Pakistan will also closely monitor designated crossing points.
The decision comes as the surge in violence in Afghanistan this year triggered a row between the two allies in the US-led "war on terror", with Kabul accusing Islamabad of trying to destabilise Afghanistan.
About 4,000 people, including 1,000 civilians, have died this year in the fighting, making 2006 Afghanistan's bloodiest year since the fall of the Taliban government five years ago.
Hamid Karzai, the US-backed Afghan president, this month for the first time publicly accused Pakistan of supporting Taliban fighters, saying elements in Islamabad wanted to turn Afghans into "slaves".
Porous border

It will be the first time Pakistan has mined its 2,400km porous border with Afghanistan.
Pakistan says it has 700 military posts
along the border with Afghanistan [AFP]
The border belt is inhabited by conservative and independent ethnic Pashtuns, who have blood relations on both sides of the frontier.
Islamabad had proposed a similar move before, but the Afghan government opposed the idea.
"We don't need any agreement from any country to fence or to do whatever measures we need to take on our side of the border," Khan said.
In his attack, Karzai had accused the Pakistani military intelligence, the Inter-Services Intelligence (ISI) in particular, of helping the Taliban.
Pakistan denies the charge, saying it has 80,000 soldiers along the border with Afghanistan, hundreds of whom have died fighting pro-Taliban fighters.
Khan said Pakistan also maintained more than 700 posts along the Afghan border.
"Of course the responsibility for interdicting or preventing any such militancy is not just the responsibility of Pakistan side, but is equally the responsibility of the ISAF (International Security Assistance Force), Nato and the Afghan forces," he said.
Pakistan has also decided to monitor its Afghan refugee camps.
Khan said: "We also request the international community especially the United Nations to expedite the refugees' return to Afghanistan and to relocate some of the camps which are closer to the border."