Though the new fence threatens to block the route again just as winter is approaching, migrants have largely been able to find their way around such obstacles. Officials are anticipating that asylum seekers may now turn to Albania and Italy as an alternative routes to more prosperous European countries in the north.

Most of the migrants moving through the Balkans have gone on to Austria and then Germany. Slovenia has been saying that it could manage the influx of about 6,000 to 8,000 asylum seekers a day as long as roughly the same number moved on each day. But Austria said recently that it could accept only 6,000 people a day from Slovenia, and Germany has started to tighten some controls on arrivals.

As a result, the Slovenian prime minister, Miro Cerar, said that it was very likely that many of the 30,000 migrants who were traveling north from Greece could become stranded in Slovenia, which has limited room to accommodate them.

“It’s a big number,” Mr. Cerar said at a news conference on Tuesday, after the government approved construction of the fence. “If we don’t act now, we could have a humanitarian catastrophe on the territory of Slovenia,” he said.