A missile defence system covering all Nato countries has been agreed at the alliance's summit in Lisbon, US President Barack Obama has said.
The shield would cover all Nato members in Europe and North America.
Nato is also backing the swift ratification of the Start treaty between the US and Russia, cutting nuclear weapon stockpiles.
The summit will discuss Afghanistan on Saturday, with plans to end combat operations by 2014.
Nato will ask Russia to co-operate on missile defence and join the system.
On Saturday, leaders will meet Russian President Dmitry Medvedev, who is attending the summit.
Discussions will also focus on a shift in strategy for Nato in Afghanistan.
Proposed Nato missile defence plan
Missile defence has been a point of tension between Nato and Russia in the past, but President Medvedev's presence at the summit is being seen as a strengthening of ties.
"It offers a role for all of our allies. It responds to the threats of our times," Mr Obama said, according to the Reuters news agency.
"Tomorrow we look forward to working with Russia to build our co-operation with them in this area as well, recognising that we share many of the same threats," Mr Obama said.
Last month, President Medvedev said Russia was considering whether to join a missile shield.
In November 2009, President Obama announced the US was scrapping plans for a missile shield based in Poland and the Czech Republic which had infuriated Russia.
Nato Secretary General Anders Fogh Rasmussen and Mr Obama both called for the speedy ratification of the Start treaty, which Russia and the US signed in April.
The agreement would reduce both countries' nuclear arsenals and allow each to inspect the other's facilities.
Any delay "would be damaging to security in Europe," Mr Rasmussen said.
The treaty is currently before the US Congress, where some Republican lawmakers are resisting ratification, saying they need further reassurance about America's nuclear deterrent capability after Start.
The Obama administration currently needs eight Republican votes in the Senate to reach the 67 out of 100 needed to ratify the treaty.
But that number will increase to 14 when the new Congress convenes in January, as the Republicans won more Senate seats in the mid-term elections.
On Saturday, the discussion will focus on Afghanistan, where President Obama has said Nato is "moving to a new phase".
He said it was the beginning of "a transition to Afghan responsibility that begins in 2011 with Afghan forces taking the lead for security across Afghanistan by 2014".
Afghan President Hamid Karzai, who is scheduled to address the summit on Saturday, has said he wants Nato to hand back control of the country's security by the end of 2014 - a deadline the US has described as realistic but not set in stone.
The BBC's Paul Wood in Kabul says the last thing Nato wants is for its military campaign to end with parts of Afghanistan in the hands of warlords and an opium mafia.
Nato generals do not use the term "victory" any more.
But our correspondent says if the violence is at a level that can be managed by Afghan forces, Nato will consider it has done its job - and the troops will start coming home.