Russia, Armenia, Belarus, Kazakhstan, Kyrgyzstan and Tajikistan decided to turn the military alliance uniting them since 1992 into a formal organization, they said.
The decision was reached at a Kremlin summit hosted by Putin and attended by the president of Armenia, Robert Kocharian, Belarus' Alyaksandr Lukashenka, Kazakhstan's Nursultan Nazarbayev, Kyrgyzstan's Askar Akayev and Tajikistan's Emomali Rakhmonov.
But at least one analyst said the new structure, which came as NATO and Russian foreign ministers met in Reykjavik to formalize new cooperation, was mostly designed to appease the Russian political and military elite worried by this rapprochement and the presence of U.S. forces on former Soviet soil.
The agreement reached "is meant to appease Russian conservatives who criticize Putin's foreign policy for being too pro-Western," said Andrei Piontovsky, who heads the Moscow-based Center for Strategic Research.
Putin's hawks "cannot forgive him for having given the green light to the deployment of U.S. troops in Central Asia," he added.
U.S.-led anti-terrorist coalition forces are stationed in three ex-Soviet republics bordering or near Afghanistan - Tajikistan, Uzbekistan and Kyrgyzstan.
Kyrgyzstan and Tajikistan are members of the new military organization.
One hundred and fifty U.S. instructors are also expected in Georgia, where they are to train local forces in the fight against terrorism.
However, Putin was careful to tone down the new organization's anti-Western stance, saying that it could work together with NATO.
"This organization can become an important element of global security and is prepared to cooperate with NATO," the Russian president told reporters following the Kremlin summit.
The Russian president added that the structure had not been created to offset NATO.
Member countries "are not united against someone, but against threats," he said.
But setting up the organization was also a way for Moscow to reassert its influence over once-Soviet Central Asia.
"This deal also allows Russia to show it intends to preserve its influence in the former Soviet republics, even though it cannot afford it anymore," Piontovsky said.
However, the fact that the six leaders could not reach an agreement on an integrated command center for their armed forces could undermine the credibility of Moscow's endeavors.
The six countries said they would set up a commission before July 1, which would elaborate documents giving the alliance organization status. The documents should be ready on Nov. 1.
Putin said the new body had been set up to react to a fast-changing global context and could in the future cooperate with other security organizations. "We are living in a fast-changing world and, therefore have to reinforce the treaty linking us and to adapt it to new threats," he said.