At least seven people were killed as heavy rains triggered flash floods in northwest Turkey, state news agency Anadolu said on Wednesday.

Five people died and one person was missing in Kirklareli province near the border with Bulgaria and Greece, after houses were flooded late on Tuesday, local media reported.

Authorities said a further two people were killed and 31 injured in Istanbul's northern Basaksehir and Kucukcekmece districts, where images showed cars floating in floodwaters.

Interior Minister Ali Yerlikaya said search and rescue operations were continuing and pledged financial support to families in flood-affected areas.

Around 125 kilograms of rain per square metre hit Istanbul in less than six hours, equal to what Turkey's largest city would typically expect in September as a whole, the governor's office said in a statement.

Torrential rain has caused unprecedented flooding across southeast Europe, damaging infrastructure and prompting evacuations.

At least two people died after homes and businesses were flooded in central Greece on Wednesday. Two were killed by floods in Bulgaria's Black Sea region on Tuesday.