The UN Refugee Agency says more than 400 refugees and migrants on board three rescue ships in the Mediterranean must be allowed to disembark safely.
A joint statement issued with the International Organization for Migration said it was a “humanitarian imperative to save lives”.
Italy’s coastguard has now evacuated 49 people from an overloaded rescue vessel funded by British artist Banksy.
Overnight its crew had tweeted a call for immediate assistance.
European authorities had not responded adequately, they said.
The UNHCR and the IOM said the lack of a deal on a regional landing system could not be an excuse to deny vulnerable people safe harbour.
“It is essential that other EU member states provide more support to countries at the forefront of welcoming those arriving by sea in the Mediterranean,” the joint statement said.
The Louise Michel, which is funded by Banksy, has already carried out a number of rescue missions.
Its crew tweeted that they had managed to keep the vessel stable and had a dead body of a would-be migrant in its only life raft.
The UNHCR and IOM said there were more than 200 rescued refugees and migrants on the Louise Michel. Italian state broadcaster RAI put the number at 130.
“They must be urgently transferred and disembarked from the Louise Michel, which is currently well beyond its safe-carrying capacity after carrying out a sunrise rescue this morning,” the UNHCR and IOM said.
“Any delays could jeopardise the safety of people on board, including the crew.”
On Saturday, the Louise Michel confirmed via Twitter that the Italian coastguard had taken 49 of the most vulnerable survivors but this left “the majority still waiting”.
It also said another rescue vessel, the SeaWatch4, which is much larger but already has 200 migrants on board, had arrived to “assist us doing what Europe falters to do”.
The SeaWatch4 later tweeted it had taken the remainder of the Louise Michel’s migrants on board but was now carrying 350 people who needed to be allowed to come to shore as soon as possible.
A third vessel also needs assistance. A group of 27 migrants and refugees, including a pregnant woman and children from Libya, have been aboard the Maersk Etienne since their rescue on 5 August.
“A commercial tanker cannot be considered a suitable place to hold people in need of humanitarian assistance or who may be in need of international protection,” the UNHCR and IOM said.
According to UN data, 443 people have died or have gone missing trying to cross the Mediterranean from north Africa so far in 2020.
Just over 40,000 have arrived in Europe by sea during the same period.