BUDAPEST (Reuters) - Hungary's government will start preparatory work to extend the border fence along its Serbian frontier eastwards towards Romania, in case migrants start taking other routes into its territory, the foreign minister said on Tuesday.

Hungary has already been building a 175-km (110-mile) razor wire fence on the Serbian boundary, across the main land route for migrants into the EU.

"We have made the decision to start preparatory works for the construction of a fence starting from the Hungarian-Serbian-Romanian border at a reasonable length should migration pressure shift in the direction of Romania," Foreign Minister Peter Szijjarto told a news conference.

Hungarian Prime Minister Viktor Orban, one of the continent's loudest opponents of mass immigration, says he is acting to save Europe's "Christian values" by blocking the main overland route used by mainly Muslim refugees, through the Balkans and across his country via its border with Serbia.

Hundreds of thousands of people have been arriving at the EU's southern and eastern edges and making their to the richer countries further north and west, in the greatest migration to Western Europe since World War Two.

Szijjarto said Budapest had informed the Romanian Foreign Ministry about the plans, which would extend the fence's current route to a few kilometers beyond the Maros (Mures) river along the Romanian frontier.

(Reporting by Gergely Szakacs; Editing by Andrew Heavens)