Hungarian soldiers started building a fence Monday on the border with Serbia, an effort meant to stop the rising flow of migrants trying to enter the European Union.

On the outskirts of the southern village of Asotthalom, soldiers were using heavy machinery to drive metal rods into the ground, the first steps in the construction of the nearly 4-meter high fence, which the government wants to finish by Aug. 31 along the 175-kilometer.

On July 16, government officials had presented a 150-meter sample section of the fence at the border, built to test different construction techniques and materials, but construction began in earnest on Monday.

Work on the fence is being carried out at several locations at once, with around 900 soldiers taking part in the project. Some elements of the fence, including the razor wire to be placed on top of the barrier, is being prepared by inmates from Hungarian prisons, and people in a state work program may also be sent to help the soldiers.

More than 100,000 migrants have reached Hungary on routes across the Balkans so far in 2015, compared with fewer than 43,000 asylum seekers last year and 18,900 in 2013.

The initial sections of the fence will be built in the areas, like those near Asotthalom, which are most heavily used by human smugglers, blamed by Hungarian authorities for the jump in the number of migrants.

In the latest case, prosecutors said Monday they had arrested five people, including three police officers, suspected of smuggling migrants in southern Hungary.

Between Friday and Sunday, nearly 4,500 migrants were caught entering Hungary without permission.

While earlier most of the migrants were from Kosovo, over the past several months about 80 percent of them are from war zones like Syria, Iraq and Afghanistan. Nearly all request asylum in Hungary but try to move to EU countries further west like Germany, Sweden or Britain before their claims are settled.

Ahmed Saad, a refugee from the Yazidi Kurdish community in Iraq, said his group had walked across Turkey and Bulgaria from their hometown of Sinjar.

"We left all - my job, all my car, my house, all things, because we ran going to Germany," Saad said near Asotthalom shortly after crossing into Hungary.

From this month, the Hungarian government has also enacted tougher migration rules like speedy procedures to determine the merit of asylum requests and the possibility to detain refugees for longer periods.