Thick black smoke billowed Thursday from the chimney of the Sistine Chapel in a sign that cardinals again failed to elect a new head of the Catholic Church.

Among the thousands of Catholics and curious tourists massed in St Peter's Square, there was applause but also sighs at the result, which came after the second and third ballots.

The 133 cardinals voting for a successor to Pope Francis as head of the world's 1.4 billion Catholics were shut in for the secretive conclave on Wednesday evening.

Sequestered away from the world, they communicate their progress by burning their ballot papers and sending up smoke through the chimney of the frescoed 15th-century chapel.