BUDAPEST—Hungary will build a second, massive fence along its border with Serbia to stop an influx of migrants in case an agreement with Turkey to contain them fails, Hungarian Prime Minister Viktor Orban said.

Hungary hopes talks with Turkey to contain the migrants will succeed but it must prepare now in case they fail, Mr. Orban said in an interview on state radio.

"I wish this won't happen. But we have to prepare for a situation where the Turks' policy changes and suddenly several hundreds of thousands of migrants appear all at once at Hungary's borders. And then, if it doesn't work with kind words, we'll have to hold them back with force, and that we shall," Mr. Orban said.

To help block migration, the Turkish government must be stable, with its President Recep Tayyip Erdoğan safe in his position and a predictable foreign policy, the Hungarian leader said.

Hungary already built a hastily raised fence with razor wire along its border with Serbia and Croatia last year to deter migrants mostly from the war-torn regions of the Middle East and Afghanistan from entering the passport-free zone of the European Union through its borders.

The move has shifted the flow of people amid the biggest migration wave the continent has faced since World War II. Migration undermines public safety, brings terrorism and is a threat to the European way of life, Mr. Orban said.

The current Hungarian fence can deter a few thousand or tens of thousands of migrants but not more, Mr. Orban said. The new fence being planned will be a massive defense system built with the latest technologies, he added. Hungary has about 174 kilometers (108 miles) of border with Serbia. Hungary will also increase the number of police patrolling its southern borders by 3,000, boosting its total police force to 47,000, Mr. Orban said.

Mr. Orban made the announcement before a meeting on Friday in Warsaw with leaders of Slovakia, the Czech Republic and Poland, and German Chancellor Angela Merkel. Their discussions will include Europe's stance on migration in preparation for an EU summit in September.

The meeting should show whether Ms. Merkel backs the four central European countries in their rejection of migrants to be settled in their territories under an EU distribution regime, Mr. Orban said.

"Borders cannot be protected with flowers and stuffed animals. They can be protected with fences, soldiers and arms, and we surely will defend them," he said.

Write to Margit Feher at margit.feher@wsj.com

(END) Dow Jones Newswires

August 26, 2016 05:55 ET (09:55 GMT)