Spanish campaigners said on Thursday they have written to German Chancellor Angela Merkel demanding she apologise for her country helping the ex-dictator Franco in Spain's civil war.

Merkel is due to land in Spain on Sunday for a two-day visit for meetings with Spanish Prime Minister Mariano Rajoy in the northwestern pilgrimage city of Santiago de Compostela.

The visit drew the attention of the Association For Recovering Historical Memory (ARMH), which honours people killed by Franco's forces in the 1936-1939 war and subsequent dictatorship.

It demanded an "apology" from Merkel for Nazi Germany's support for Franco, according to the article, penned by news outlet El Diario and posted by the ARMH on its website and Twitter feed.

The association wrote to Merkel asking her to "take advantage of her visit to Spain to settle the historical debt for the crimes of the Condor Legion", referring to the German contingent sent by Hitler to back Franco's uprising against Spain's Republican government, which helped him win the civil war.

"The country you lead has an enormous debt to the victims of the Franco dictatorship," the association told Merkel in its letter.

Germany's most notorious act in the Spanish civil war was when its war planes bombed the Basque town of Guernica -- an atrocity immortalised in Pablo Picasso's classic painting.

Merkel and Rajoy have been allies in steering Spain out of a recent financial crisis which threatened the whole eurozone, and in stabilising the finances of the heavily indebted country.

Rajoy passed painful economic reform measures under pressure from Germany and other European partners, which turned many Spanish protesters against Merkel.