With Russian President Dmitry Medvedev and Abkhazia's leader Sergei Bagapsh looking on, their defence chiefs signed an agreement allowing Moscow to build a military base in the breakaway territory on the Black Sea.
The base will accommodate at least 3,000 Russian land troops, already stationed in Abkhazia, for at least 49 years, Abkhaz officials said.
"This agreement creates a foundation for the development of Abkhazia as an independent state," Medvedev said at a signing ceremony in the Great Kremlin Palace.
Moscow recognised Abkhazia in August 2008 after crushing an assault by US ally Georgia on another pro-Russian breakaway region, South Ossetia, in a five-day war.
The regions, which broke away from Georgia in bloody wars in the early 1990s, are almost completely dependent on Russia. Both use the Russian rouble and Moscow has issued most residents with Russian passports and pays their pensions.
The new base is one of several that Russia plans to build on the sliver of land, which is close to NATO member Turkey, in the near future. The others are for airborne troops and the navy.
NATO reacts swiftly
NATO swiftly called on Russia to reverse the deal, saying it violated the restrictions on military forces stipulated in the EU-mediated ceasefire agreement after the 2008 war.
"We do not consider any agreements signed between the Russian Federation and the regions of Georgia as having any validity," the alliance's spokeswoman Carmen Romero said.
Georgia had decried the plans for a land base as illegal and called it part of Moscow's "occupation" campaign.
In April 2009, Russia formally took control over the de-facto borders of Abkhazia and South Ossetia, provoking condemnation from Western powers.
The European Union and NATO have repeatedly expressed concern that a Moscow-led military build-up in Abkhazia threatens Georgia's territorial integrity.
It is also watched with unease by Western powers for its proximity to crucial energy routes which flow to the EU.
Medvedev said the base would not violate Russia's international obligations.
(EurActiv with Reuters.)