ROSTOV-ON-DON -- Russian border guards have begun defending the de facto borders between Georgia and its breakaway regions of Abkhazia and South Ossetia.

The deployment, announced Saturday, quickly followed the signing of a deal that formalized Russia's control over the borders of the regions at the center of last summer's brief war with Georgia.

The border guards department for southern Russia said its troops took full responsibility for guarding the borders of South Ossetia and Abkhazia on Thursday, the same day that the leaders of both regions signed an agreement with President Dmitry Medvedev giving Russia control over the borders. The first border guards arrived in South Ossetia on Thursday, it said.

"The border must be securely closed and made inaccessible for enemies," said Nikolai Lisinsky, who heads the border guards department in southern Russia, Interfax reported.

He said video surveillance and unmanned aircraft would be used to patrol the border, but no barbed-wire fences would be put up and South Ossetia would remain accessible "for those who seek peace with us."

In Abkhazia, the region's border service said Russian border guards were deployed together with Abkhaz troops along the land borders. Abkhaz troops continued to guard the Black Sea coast, although Russian naval vessels were deployed off shore, the service said.

Russia has announced plans to start building a naval base in Abkhazia.

In signing Thursday's deal with Abkhaz leader Sergei Bagapsh and South Ossetia leader Eduard Kokoity, Medvedev indicated that Russia's intention was to strengthen its position that the cease-fire that ended last summer's war had been superceded by subsequent agreements with South Ossetia and Abkhazia. "This, without any doubt, is a political act," Medvedev said at a Kremlin ceremony. "These documents develop the agreements on friendship, cooperation and mutual assistance that were signed some time ago in this hall."

Georgian President Mikhail Saakashvili condemned Thursday's deal.

"We are seeing some kind of legal maneuvering to try to legalize, but you cannot legalize something that is fundamentally illegal," he told reporters in Warsaw. "It is very dangerous to everybody, including Russia itself."

The United States also expressed "serious concern" over the deal.

"This action contravenes Russia's commitments under the Aug. 12 cease-fire agreement brokered by French President Nicolas Sarkozy," State Department spokesman Robert Wood said in a statement Friday.

Wood said the United States wanted Russia not only to abandon the agreements but to relocate its own troops to positions held before the conflict last August.

Medvedev suggested that Georgia's Western partners were jeopardizing the cease-fire through actions that could be seen by the Georgian government as supporting its efforts to rebuild its military. He pointed specifically to NATO military exercises beginning in a few days in Georgia, calling them an "open provocation." "Military exercises should not be held where relatively recently there was a war," Medvedev said.