Hungary has announced plans to build a large fence to prevent border crossings from Serbia.

AFP reports the "border barrier" will run to 175 kilometres and be four meters high.

Serbian Prime Minister Aleksandar Vucic said he was "surprised and shocked" by the plan. "We will discuss this decision with our Hungarian colleagues," he said.

The Hungarian Minister for Foreign Affairs Peter Szijjarto says the decision - which aims to cut down on the numbers of migrants - does not break any international rules.

Hungary says 95 per cent of the migrants who have entered the country so far his year have entered through Serbia.