US President Barack Obama and Russian President Dmitry Medvedev had an unplanned one-on-one meeting on the sidelines of a NATO summit in Lisbon, a White House official announced.

"They were able to go aside into a room by themselves and talk for 15 to 20 minutes," White House spokesman Ben Rhodes told reporters aboard Air Force One as it was flying to Washington. "It was informal, it wasn’t planned."

NATO and Russia agreed earlier yesterday to jointly examine a missile shield to defend Europe and boost the flow of supplies to the Afghan war, burying a period of tensions between the former Cold War foes.

The day before, the Western military alliance had agreed to set up an anti-ballistic missile defence system to protect Europe's populations against rogue rocket attacks and invite Russia to cooperate.

The two sides agreed to study how this cooperation would work, but Medvedev warned Russia would have to be treated as an equal partner if it is to participate.

Rhodes said Obama and Medvedev had developed what he called "a very strong rapport."

"They like each other. They like to see each other," the spokesman said. "They like to get along and consult about things."

According to administration officials, the meeting was initiated by Obama, who later described it as "very cordial."

Obama also talked to Medvedev about his earlier meeting with President Mikheil Saakashvili of Georgia.

Georgia fought a five-day war with Russia in August 2008 when Russian troops poured into the country after fighting broke out between Georgian and separatist forces in South Ossetia.

One of the officials quoted Obama as saying that the meeting with Medvedev was aimed at reducing "misunderstandings that can lead to unintended consequences."

The two leaders also discussed the new START nuclear arms treaty with Russia and the chances for its early ratification by the US Senate.

"They had a very cordial conversation about it," an administration official said, adding that Medvedev expressed confidence "in the president getting it done."

The treaty -- signed by Medvedev and Obama at an elaborate ceremony in Prague in April -- restricts each nation to a maximum of 1,550 deployed warheads, a cut of about 30 percent from a limit set in 2002.

The agreement, a top Obama foreign policy initiative, replaces a previous accord that lapsed in December 2009 and also requires ratification by Russia's lower house, the Duma.

Senate Republicans have said they need to be sure that the US nuclear arsenal will be modernized and that the treaty will not hamper US missile defense efforts -- but some acknowledged privately that they did not want to hand Obama a major diplomatic victory before the elections.

Medvedev's participation in Lisbon marked the first meeting between a Russian president and the 28-nation alliance since his country's 2008 war with Georgia, a pro-Western former Soviet state that now aspires to join NATO.