Two paintings, one by art master Pablo Picasso and another by famous Dutch painter Piet Mondrian, were stolen from Greece's National Gallery on Monday, police sources said.
The robbers broke into the gallery in the early hours and snatched the 1939 Picasso work "Woman's head", a police official said without disclosing the other artwork's title.
The Culture Ministry and police confirmed that two paintings had been stolen but would not officially reveal the names of the artists.
"After the alarm went off the guard discovered that the two paintings were missing. Another was lying on the floor," an official told Reuters.
Police were still investigating if any other art is missing.
The National Gallery's collections include two Mondrian paintings, the "Mill" and the "Landscape" both dated 1905 and a drawing, the "Study of Flower."
In October, police in Serbia recovered two paintings by Picasso stolen in 2008 from a gallery in Switzerland and worth millions of dollars.
Greece recovered in September a painting by Flemish master Peter Paul Rubens stolen from a museum in Belgium in 2001 and arrested two Greeks who tried to sell it to undercover police for about one million euros.