US President Barack Obama has offered his strongest condemnation yet of WikiLeaks' "deplorable" documents dump, as supporters of Julian Assange called for demonstrations to press for release of the website's founder.

In a call to Turkish Prime Minister Recep Tayyip Erdogan, Obama "expressed his regrets for the deplorable action by WikiLeaks and the two leaders agreed that it will not influence or disrupt the close cooperation between the United States and Turkey," the White House said.

Supporter of Julian Assange protest in front of the British Embassy in Madrid. Credit:AP

The comments, and similar statements in a call to his Mexican counterpart Felipe Calderon, were Obama's most forceful yet against the website, whose disclosure of a trove of secret US diplomatic cables has won it both condemnation and praise.

Obama's call to Erdogan could be seen as an effort to soothe ruffled feathers in Turkey - a key regional US ally - where officials including the prime minister have railed against some of the information divulged by the documents.