Pakistan, its ties with powerful ally the United States heavily strained, is looking increasingly isolated after rival India signed a wide-ranging agreement with neighboring Afghanistan.
"Pakistan is our twin brother, India is a great friend. The agreement we signed with our friend will not affect our brother," Karzai said in a speech in New Delhi.
"This strategic partnership ... is not directed against any country ... this strategic partnership is to support Afghanistan."
On a two-day visit to New Delhi, Karzai and Indian Prime Minister Manmohan Singh sealed an agreement on Tuesday that spanned closer political ties to fighting terrorism.
It signals a formal tightening of links that may spark Pakistani concerns India is increasingly competing for leverage in Afghanistan. The agreement with India is one of several being negotiated by Kabul, including one with the United States, that are part of an Afghan bid for greater security as NATO troops head home.
The strategic partnership sealed with India on Tuesday — the first such pact between Afghanistan and another country — deepens already friendly ties and aims to boost trade, security and cultural links.
But Indian involvement in Afghanistan is extremely sensitive because of the delicate and often deadly power games in South Asia, with Pakistan vehemently opposed to its arch-foe meddling in what it considers its backyard.
"The signing of the strategic partnership with India is not directed against any country. It is not directed against any other entity. This is for Afghanistan to benefit from the strength of India," Karzai added.
New Delhi, fearful of the return of a Taleban regime in Kabul, has plowed more than $2 billion of aid into Afghanistan to gain influence, helping fund highways and the new national parliament building.
Karzai stressed that the focus of his efforts in bringing peace to his war-torn country, where international forces are set to withdraw from 2014, would now be on talking to Pakistan.
He added, however, that engagement with Islamabad had "unfortunately not yet received the result that we want."
Relations between the two nations are severely strained after Karzai accused his neighbor of playing a "double-game" by covertly funding militant groups that carry out attacks in Afghanistan.
The assassination of Kabul's peace envoy to the Taleban Burhanuddin Rabbani last month forced Karzai to re-examine his long-running strategy of trying to broker contacts with the Taleban to open peace negotiations.
"We have now decided not to talk to the Taleban because we don't know their address. When we find them, we will talk to them. Therefore we have decided to talk to our brothers, our neighbors, in Pakistan," Karzai said.
Analysts in India had predicted that Karzai, angry at Pakistan and wary of a drawdown of US troops by 2014, would look to elevate India's role in stabilizing his war-torn country.
The strategic agreement includes pledges by India to help train and equip Afghan security forces and improve two-way trade, with both countries promising to work for "everlasting peace and friendship between the two governments."
