BARCELONA (Reuters) - A Spanish charity boat which rescues migrants at sea has been blocked by authorities from operating in the Mediterranean, the non-governmental organization said on Monday.

FILE PHOTO: NGO Proactiva Open Arms' rescue boat is seen docked with migrants rescued in central Mediterranean Sea, at the Center for Temporary Assistance to Foreigners (CATE) in the port of Algeciras, in Campamento, Spain December 28, 2018. REUTERS/Jon Nazca/File Photo

The Barcelona Maritime Authority, in a letter seen by Reuters, said the charity Proactiva Open Arms had violated international agreements that rescued migrants should be taken to the closest port available.

The minority Socialist Party government has accepted hundreds of migrants rescued by Open Arms this year but anti-immigrant sentiment is on the rise in Spain, which has seen a surge in people making the dangerous crossing by sea.

Open Arms, which operates in the waters between southern Europe and Libya, has previously traveled to Spain to disembark migrants after being refused permission to do so in Italy and Malta.

“Measures were not taken to ensure the disembarkment took place as quickly as reasonably possible. In contrast, the boat had to wait with a large number of people on board and subsequently travel for several days crossing the Mediterranean to arrive to a safe port,” the letter reads.

Open Arms will not be granted permission to resume rescue operations “until an agreement with the authorities of the relevant SAR [search and rescue] zones is in place,” the letter continued.

The Barcelona Maritime Authority, which is part of the Ministry of Public Works, could not be immediately reached for comment.

Open Arms said on Monday it had appealed the decision.

“Preventing us from saving lives is irresponsible and cruel. Cowardly politicians have set the counter of deaths running,” Open Arms founder Oscar Camps wrote on Twitter on Monday.

As of November last year, more than 600 African migrants had died trying to reach Spain since the start of the year, almost three times more than in all of 2017.

Irregular immigration by sea has fallen on average across Europe, though arrivals to Spain have more than doubled annually to the rate of more than 1,000 people per week this year.

In December, Vox became the first far-right, anti-immigrant party to win representation in a regional assembly in Spain since the end of the Franco dictatorship in the late 1970s.