BUENOS AIRES (AFP) - Rescuers on Thursday searched for some 20 people still missing in the Argentine city of La Plata after 57 were killed in a violent storm and heavy flooding.

Most of the victims were found on Wednesday after a second day of record rainfall in Buenos Aires and nearby La Plata, where flooding submerged cars and sent people scrambling to rooftops for safety.

At least 49 people died in La Plata, a bustling university city of under one million about 60km south of the capital, authorities said, raising the toll by one on Thursday. Six people died in Buenos Aires proper and two others in the city's suburbs.

"We are carrying out the final count because there are still about 20 people who have not been found," said Argentine deputy security minister Sergio Berni.

He said that authorities had received no new calls for rescue in the past hours.

Of those killed in La Plata, provincial governor Daniel Scioli said that 34 have been identified.

Residents of the most heavily flooded neighbourhoods trickled back home after a staggering 40cm of rain fell on La Plata during a two-hour period from late Tuesday into Wednesday, knocking out phone and power lines and leaving about half the city in the dark.

Flood waters reached 2m in some places, turning city roadways into raging rivers.

"I heard piercing screams, I saw bodies float by. Nobody came to help, not a firefighter, or a policeman, or a soldier," an outraged La Plata resident told local TV news, angry at the lack of official aid.

President Cristina Kirchner, who spent much of her childhood in La Plata and surveyed the devastation by helicopter on Wednesday, decreed three days of national mourning starting from Thursday.

In Buenos Aires, more than 15cm of rain - an April record - fell between late Monday and early Tuesday, the weather service said.