Reporters examine a bombed Georgian railway bridge on Saturday. (Mike Hornbrook/CBC)Russian troops blew up a railway bridge on Georgia's vital main rail line Saturday after Russia's president signed a ceasefire agreement designed to end the fighting, Georgia's vice-prime minister says.

Giorgi Baramidze led a group of journalists to view damage to the bridge, which he said was blown up about noon local time on Saturday.

The Georgians said they have pictures of Russian soldiers planting the explosives, the CBC's Mike Hornbrook reported. They also said they discovered explosives at two other key bridges, but were able to defuse them. Russia denies any involvement.

Georgia's vice-prime minister, Giorgi Baramidze, speaks with a reporter on a bombed rail bridge Saturday. (Mike Hornbrook/CBC)The bridge is on the sole rail line that serves as a vital freight and passenger link for Georgia, the Caucasus and western Asia. Its loss is a huge economic blow to Georgia and the whole region, Hornbrook said.

Baramidze said it will take at least 15 days and perhaps longer to restore the bridge.

Georgia also accused Russia of setting forest and grass fires across the country. It said the blazes could burn electricity transmission lines and cellphone towers, leading to widespread power and phone outages.

Earlier, a Kremlin spokesman said Russian President Dmitry Medvedev had signed a ceasefire agreement to end fighting in Georgia over the breakaway province of South Ossetia.

Medvedev spokesman Alexei Pavlov said the president signed the order on Saturday, but did not give further details.

The move comes a day after Georgian President Mikhail Saakashvili signed the plan that calls for Russian troops to pull back, but that also grants them limited patrols inside Georgia, in both South Ossetia and Abkhazia, another breakaway province with close ties to Russia.

Georgia's Foreign Ministry claimed Saturday that Russian-backed separatists from Abkhazia have seized 13 villages in Georgia and a power plant.

A ministry statement said Russian army units and separatist militants shifted the border of Abkhazia toward the Inguri River. It said they set up temporary administration in 13 villages and put the Inguri hydropower plant under separatist control.

The claim could not immediately be independently confirmed.

The French-brokered ceasefire plan appears to leave some tense issues open to interpretation. They include whether Georgia will be able to send troops back into parts of South Ossetia, where intense fighting between Georgian and Russian troops erupted on Aug. 8.

Georgia had launched a massive barrage to try to take control of the Russia-backed separatist territory, prompting the Russian army to send in troops and tanks that quickly overwhelmed the smaller country's forces.

Russian soldiers jump from a truck in Igoeti, northwest of the capital Tbilisi, Georgia, on Saturday. (Darko Bandic/Associated Press)

Earlier in the week, South Ossetia's leader, Eduard Kokoity, said no Georgian peacekeepers or international observers would be allowed in South Ossetian territory.

Russian troops described by Moscow as peacekeepers remained on the ground in parts of Georgia on Saturday.

Local residents reported seeing Russians in shallow foxholes, 50 kilometres from Tbilisi, in the centre of Igoeti. Although they later abandoned their positions, their tanks were still in place on a hillside on the edge of the town.

About 30 kilometres away along the same road, Russian forces still controlled the strategic city of Gori, 15 kilometres south of South Ossetia, presumably to maintain access to the country's major east-west highway.

With files from the Associated Press