Russia has vetoed an extension of the UN observers' mandate in Georgia, so a 131-strong UN team will now have to leave the Georgia-Abkhazia border zone.
Russia's move at the UN means there will be no more impartial international observers inside the disputed regions of Abkhazia and South Ossetia.
The UN mandate was "built on old realities," said Russia's ambassador to the UN, Vitaly Churkin.
Russia has controlled the rebel areas since a war with Georgia last year.
Speaking at the UN Security Council in New York, Mr Churkin said "only a new security system on the Georgian-Abkhaz border could guarantee non-aggression by Georgia".
Russia has recognised the independence of Abkhazia and South Ossetia, but the West says the two regions remain part of Georgia.
Abkhazia's ethnic Georgian community complains that security has deteriorated since last year's war.
Georgia's Foreign Minister Grigol Vashadze denounced the Russian veto, saying Tbilisi would never accept any decision that challenged Georgia's territorial integrity.
UK Foreign Secretary David Miliband accused Russia of using its veto power to "pursue its own narrow interests".
The UN mission has been on the ground in Abkhazia since 1993, when it was deployed to monitor an earlier ceasefire between Abkhaz separatists and Georgian forces.
The BBC's Tom Esslemont in Tbilisi says the ending of the UN mandate comes just as the Organisation for Security and Co-operation in Europe (OSCE) prepares to terminate its small observer mission in Georgia too.
It means that the European Union Monitoring Mission is the only recognised international observation force on the ground, he reports.
With Russian soldiers and Georgian police units stationed metres away from each other along the borders, it now falls to the EU monitors alone to make sure the fragile truce is honoured. But their mandate only allows them access to areas under Georgian control, our correspondent says.