The head of Hungary's immigration office says it has received 110,000 asylum requests so far in 2015, a development that has prompted the government to start building a fence on the border with Serbia.

Zsuzsanna Vegh said registered asylum seekers were directed to one of Hungary's refugee centers to await decisions over their requests, but that most left the country for other destinations before their cases were settled.

Vegh said Monday on state radio that they were now sheltering some 4,500 refugees, double their normal capacity.

Babar Baloch, spokesman for the U.N.'s refugee agency in Central Europe, said the number of families with young children among the refugee groups was on the rise.

The 175-kilometer (109-mile) fence, which began construction last week, is scheduled to be finished by Aug. 31.