BELGRADE, Serbia (AP) — Hungary's prime minister vowed Monday to protect his country's borders against a possible surge in migrant arrivals this fall and offered to help neighboring Serbia to do the same.

Viktor Orban described an agreement between the European Union and Turkey on stopping the mirgant influx into Europe as "fragile." He said it is in Hungary's interest to help prevent a pileup of migrants in Serbia as well.

"It cannot be ruled out at all that we will face challenges similar to those last year on this route," Orban warned.

He was referring to the massive influx of more than 1 million refugees into Europe in 2015, many of whom used the so-called Balkan route to enter the EU through Hungary. Balkan nations closed down their borders in March and the influx has decreased, but countries fear it could pick up pace once worsening weather averts the migrants from using the more dangerous Mediterranean Sea route.

Orban's government last year built a razor wire fence along its borders with Serbia and Croatia to stop migrants from entering. Orban recently announced plans to build another one to fortify the borders as part of his anti-immigration policies.

"We want to defend the results we achieved in the past years with hard work in Hungary, which is why we cannot accept or allow ... illegal entry into Hungarian territory," Orban said. "We will prevent this with every resource."

Serbia too has beefed up its border patrols with Macedonia and Bulgaria where migrants mostly enter the country after thousands got stranded unable to move on to any of the EU countries.

Prime Minister Aleksandar Vucic said Monday there are currently about 5,000 migrants in Serbia. Vucic said that most of them are so-called economic migrants, rather than refugees from Syria or other war-thorn countries.

"We have to prepare for the fall and winter," Vucic said. "We need to see what we are going to do."

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Pablo Gorondi contributed from Budapest, Hungary.