MILAN — A migrant rescue organization said Thursday that Italian authorities have blocked its ship in a Sicilian port, characterizing the move as harassment.

SOS Mediterranee's ship arrived in Sicily on July 7 carrying 180 migrants who were rescued off Libya. The group said in a statement that preventing the Ocean Viking from heading back to open water was ''blatant administrative harassment ... aimed at impeding our lifesaving work.''

Such actions, it said, discourage other migrant rescue vessels from operating in the perilous central Mediterranean Sea, where thousands of migrants have died while seeking safety or better lives in Europe. The Ocean Viking was the last ship actively carrying out rescues in the central Mediterranean off Libya.

Italian authorities cited safety reasons for keeping the ship in port, specifically that the Ocean Viking was carrying more people than its certification allows, according to SOS Mediterranee. The organization noted that the ship had passed previous inspections and waited 11 days to secure a port to disembark the rescued passengers.

"Why wasn't safety more of a concern to maritime authorities'' then, said Frédéric Penard, the group's director of operations.

Italian authorities similarly detained SeaWatch3 earlier this month. The aid group that operates that rescue ship, denounced the action based on a technical inspection as ''an excuse to block rescue ships.''

"They argue with the lack of safety of rescued persons on board,'' the group said on Twitter, ''but they themselves make sure that people are in distress for days'' without providing rescue.

International Organization for Migration spokeswoman Safa Msehli pointed to "very little, if any, life-saving capacity" recently in the central Mediterranean, which she called "the most dangerous sea crossing in the world."

"We continue to see people dying, we continue to see boats disappearing. It's really concerning to see such restrictions on the work of NGOs, and the absence of the state" in search-and-rescue operations, Msehli said.

"There is very little dedicated search-and-rescue capacity in the Mediterranean. NGOs have seen a lot of restrictions, and Ocean Viking is an example of that."

Italy has taken a hard line on private rescue boats, expressing concern that such operations encourage human traffickers in Libya to keep launching migrants toward Europe, often unseaworthy dinghies or fishing boats. Fresh concerns have also been generated by the coronavirus epidemic, after some migrants have tested positive.

Early in the pandemic, Italy declared its ports unsafe for docking by private rescue ships as it sought to contain its devastating coronavirus outbreak.

Migrant boats have been arriving independently on the Italian island of Lampedusa, with more than 800 arriving in recent days. More than 4,000 migrants have arrived so far this month, which is four times more than the number in July 2019 but far below the high of more than 23,500 who arrived on the island by sea in July 2016.

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Jamey Keaten in Geneva contributed.