Dozens of suspected Brazilian gangsters blasted into a secure vault containing millions of dollars in southeastern Paraguay on Monday, leaving a policeman dead in what officials called the "robbery of the century."

Some 50 gunmen with explosives and military-grade weaponry left the city of Ciudad del Este looking like a war zone after a two-hour overnight assault on a security firm and police headquarters, authorities and witnesses said.

A joint Paraguayan-Brazilian police operation against the fleeing assailants on the country's porous border -- near the famous Iguazu waterfalls -- killed three suspects and resulted in "at least four arrests," Paraguay's interior ministry tweeted.

Brazilian authorities confirmed there had been a deadly clash but would not give more details. "There was an exchange of fire," a federal police source for the state of Parana told AFP. "There were dead and wounded but we are not giving more information for now. We're still looking for people."

Brazilian President Michel Temer promised to support Paraguayan police "with all necessary resources." "The Brazilian government expresses solidarity with the victims of this criminal act and, especially, with the relatives of the dead Paraguayan policeman," he said.

Shocked Paraguayan officials said it was an unprecedented attack for Ciudad del Este, a key trading hub near the borders of Brazil and Argentina in an area linked to drug gangs.

The target of the raid was Prosegur, a firm specializing in safely transporting cash. Far from simply overwhelming security at the facility, the raiders caused multiple diversions around the city, setting vehicles on fire to block roads and detonating explosives near police headquarters.

Police reports said the attackers included members of First Capital Command, one of Brazil's most powerful drug gangs, which is based in Sao Paulo and has fought turf battles in Rio de Janeiro and across the country's prison system.