Lisbon: North Atlantic Treaty Organisation (Nato) leaders on Friday started a summit in Lisbon amid hopes they will overcome differences and coordinate their exit from war-torn Afghanistan, agree on a missile shield system over Europe and build closer ties with former nemesis Russia.

The 28 member states of the alliance will be later joined by leaders from more than 20 non-Nato countries.

For US President Barack Obama, who arrived in Portugal on Friday morning, the focus will be on ways to work with the large group on a satisfactory solution to the situation in Afghanistan.

"In Lisbon, we will align our approach so that we can begin a transition to Afghan responsibility early next year, and adopt President Hamid Karzai's goal of Afghan forces taking the lead for security across Afghanistan by the end of 2014," Obama wrote in an opinion piece published on Friday by the International Herald Tribune.

According to the US president, the US troop reductions will begin in July.

Also high on the summit agenda is the new Strategic Concept that will define the alliance's orientations in the next decade.

The alliance was created in 1949 as a common front between North America and Europe against the Soviet Union, but 61 years later, it is working on getting Russia, the core of the former nemesis, to work together for a Europe-wide security.

Anders Fogh Rasmussen, Nato's Secretary-General who wants to make enhanced Nato-Russia cooperation a hallmark of his tenure sees Lisbon as a crucial turning point in the alliance ties with Moscow.

"My strong sense is that Russia shares our view that the time has come to stop worrying about each other. The time has come to work together," Rasmussen said on Wednesday.

The new relations with Russia should lead to wider cooperation over Afghanistan and participation in the missile defense systems to be deployed in Europe over the next ten years

Obama has already welcomed a new role for Russia. "In Lisbon we can make it clear that Nato sees Russia as a partner, not an adversary," he said.

"[I]t forms the foundation of greater collaboration - with a role for all allies, protection for all allies, and an opportunity for cooperation with Russia, which is also threatened by ballistic missiles," Obama said.

Nato- Russia cooperation will encompass Afghanistan, counter-narcotics and 21st century security challenges, from the spread of nuclear weapons to the spread of violent extremism, he wrote.