ASUNCION, Paraguay - 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".

About 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 company and police headquarters, according to authorities and witnesses.

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 said.

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 said. "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".

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.

'Like bombs in a war'

The target of the raid was Prosegur, a company 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.

"The noise rang out around the city like bombs in a war," said one man who witnessed the attack, Antonio del Puerto.

Television images from the scene showed the Prosegur building was virtually destroyed, with rocket casings lying around and burned-out vehicles in the streets.

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.

Public prosecutor Denise Duarte said the men wore masks and spoke Portuguese, the language of Brazil.