(Adds Orban quotes, background)
BUDAPEST, Oct 17 (Reuters) - Hungary would have to "use force" at its southern border with Serbia to protect the European Union's frontier if Turkey delivers on a threat to open the gates for refugees through the Balkans towards Europe, Hungary's prime minister said.
Prime Minister Viktor Orban built a steel fence on Hungary's border with Serbia to seal off the Balkans route of migration, where hundreds of thousands of people marched through from the Middle East to Western Europe at the peak of the crisis in 2015.
The EU relies on Turkey to curb the arrival of refugees into Europe following a 2016 agreement to seal off the Aegean route after more than 1 million people entered the bloc.
Turkey, which hosts 3.6 million Syrian refugees, threatened to "open the gates" to allow those already in the country to head for Europe if the EU portrayed Turkey's incursion into northeast Syria negatively.
"The next weeks will decide what Turkey does with these people," Orban told private broadcaster HirTV in an interview late on Wednesday. "It can steer them in two directions: take them back to Syria or set them off towards Europe.
"If Turkey chooses the latter, these people will arrive at Hungary's southern border in huge masses," Orban said, adding that the EU, while criticising Ankara, should provide more funds to Turkey to help rebuild Syrian towns.
Orban, who has often been at loggerheads with Brussels over some of his policies affecting migrants, said currently there were about 90,000 people on the Balkans migration route and their ranks could swell to 100,000 shortly.
"If Turkey sets off further hundreds of thousands on top of this, then we will need to use force to protect the Hungarian border and the Serbian-Hungarian frontier and I do not wish for anyone that we should need to resort to that," Orban said.
Orban's nationalist government has forged close relations with Turkey, China and some ex-Soviet states in Middle Asia as part of an eastern opening initiative.
Turkish President Tayyip Erdogan is due to come to Budapest for a visit early next month. (Reporting by Gergely Szakacs; Editing by Toby Chopra)