Hungary began building a fence on its border with Croatia overnight to stem a flow of migrants and has deployed hundreds of soldiers and police on the frontier, Prime Minister Viktor Orban told public radio on Friday.
He said the fence will be finished on the critical 41-kilometre (25 mile) stretch of border where the two countries are not separated by a river by the end of Friday.
"We must implement the same measures as on the Serbian-Hungarian border," Orban said, adding that at this point 600 soldiers had worked on the fence, and a further 500 will be deployed on Friday and 700 more over the weekend.
Hungary, which has been the transit route for over 180,000 migrants this year, has built a 3.5-metre (10-foot) fence on its frontier with Serbia and implemented a raft of immigration laws to clamp down on migration. This has diverted the flow of migrants towards Croatia and Slovenia this week.
Police in Hungary, which is also in the EU's Schengen zone of border-free travel, said about 500 migrants had been detained after crossing into the south of the country from Croatia on Thursday.
On Wednesday, migrants trying to cross from Serbia clashed with Hungarian police at the border crossing of Roszke.
Orban said this was "an armed attack launched against Hungary and Hungarian police ... from Serbian territory."
"Serbia did not prevent this attack, which started from Serbian territory, in any form ... Serbian police were watching their Hungarian colleagues being attacked from Serbia," Orban told radio on Friday.
"We can also see from the analyses ...that this action was directed in Arabic and English, from loudspeakers, with organised media background," Orban added.
He said this showed that this was no longer simply an immigration problem, but "threat, danger and terrorism".
He reiterated that Hungary would have to use the army as well to protect its borders. Parliament will vote on the proposal to deploy the army next week.