By Marton Dunai

BUDAPEST (Reuters) - Hungary's parliament authorized the government on Monday to deploy the army to help handle a migrant crisis, granting the military the right to deploy a range of non-lethal force.

It passed a law saying the army could use rubber bullets, pyrotechnical devices, tear gas grenades or net guns, according to the text posted on parliament's website.

Hungary, a landlocked nation of 10 million, lies in the path of the largest migration wave Europe has seen since World War Two. It has registered more than 220,000 asylum seekers this year, a wave Budapest has said it would do everything to deflect.

Prime Minister Viktor Orban told parliament that police were unable to secure Hungary's borders with Serbia and Croatia - outer borders of the EU's passport-free Schengen zone - without help form the army.

"We can defend the Serbian stretch of the border," he said, adding that fortifications at the 175-kilometre (110 mile) Hungary-Serbia border were working better than expected.

Hungary completed that border fence and deployed regular patrols, leading to a drastic drop of migrants crossing that stretch of the border and entering Croatia instead. Unable to cope, Zagreb has waved the migrants on to Hungary again.

Croatia is not a member of Schengen, and the two countries have exchanged bitter words over the handling of the migrant crisis, with Budapest threatening a veto of Croatia's Schengen accession and beginning work on a border fence there too.

"We can defend the Croatian stretch but to do that we need the army to patrol together with the police," Orban said.

He added Hungary would act on its own until the EU finds common ground to stem the flow of migrants.

"Europe is rich but weak. That is the most dangerous combination possible," Orban said. "The result ... is catastrophic. Because Europe cannot defend its external borders, internal borders are shut again."

"We need to rethink many European inventions, institutions and treaties. But until we do we cannot sit idle. Until the EU states act as one, member states will be forced to go out of their way to fend off this brutal threat."

The radical nationalist Jobbik party, which has advocated even tougher measures, supported Orban's center-right Fidesz party in the vote, which passed with 151 votes to 12 against and 27 abstentions in the 199-member Parliament.

(Reporting by Marton Dunai; Editing by Dominic Evans)