First groups of migrants have started arriving in Croatia as they seek new entry points into the European Union after Hungary sealed off its border with Serbia.

About 80 people crossed into eastern Croatia after being taken by bus to the Serbian border town of Sid, following an all-night ride from the southern border with Macedonia.

Dozens of police and aid workers waited for the migrants across the border in Croatia, where they are being registered. Local media say some migrants have sought to cross into Croatia through nearby fields to avoid registration.

Afghan girl Parisa Sayed Jalil, 4, rests on her father's lap in a field, while they and others are detained by Hungarian police on horses for sneaking through Hungary's border fence with Serbia, in Asotthalom. Photo: Muhammed Muheisen)

Officials say more buses are expected to arrive in Sid later.

Migrants have avoided Croatia in the past because they must still go into Hungary or Slovenia before reaching Austria or Germany.

Meanwhile, small groups of migrants are continuing to sneak into Hungary from Serbia, despite a move by Hungarian authorities to start arresting those trying to breach the new razor-wire barrier.

Police on horseback surrounded a group of 14 Afghans, including five young girls and an elderly woman, in a field close to the fence.

Small groups of young men also walked along roads leading away from the border. One asked a passing reporter: "Is this the way to Budapest?"

The evidence of crossings came a day after Hungarian police arrested at least 174 migrants for illegally breaching the border or damaging the fence.

Further west, Austria has started selective controls of vehicles at three main border crossings from Hungary as it tries to impose some order over the stream of migrants.

Police said the controls could be extended to 10 crossings, with vehicles being stopped selectively for checks of passports and other travel documents.

Interior minister Johanna Mikl-Leitner has said Syrians and others in danger in their home countries can continue to ask for asylum in Austria. She said they will also be free to travel on to Germany, as has been the case up to now.