Merkel's conservative bloc is divided over immigration and stiff resistance to the euro zone plans could further erode her authority and threaten to unravel her coalition government three months after it took office.
The Lifeline is carrying 226 migrants.
"These ships belong to a German NGO and do not appear in the Dutch naval registers", the tweet said.
Rome is going to seize two vessels of German activists Lifeline and Seefuchs, which refers to the organization of the Sea-Eye, in order to check their legal status, said the Minister of transport and infrastructure of Italy, Danilo Toninelli.
"If for the arrogant President Macron this is not a problem, we invite him to stop the insults and to demonstrate generosity by opening the many French ports and ceasing to push back women, children and men to Ventimiglia".
"Mission Lifeline fears that a similar situation to the Aquarius. could be on the horizon", the group said in a statement, adding that the additional days at sea had health consequences for some of the Aquarius passengers.
Salvini said he had trust in the Austrian EU presidency to make a difference in discussions about changing the Dublin accords, noting "the mood has changed", but also hinting that Italy would be willing to play hardball, and hold back payments to the EU, if significant changes were not made.
Merkel said the ESM's crisis-response tools will be strengthened, including for "asymmetric shocks" to economies, and the fund will serve as a backstop for the EU's banking union. "Human lives must be saved, but it must be done legally and safely".
Uncertainty for Erdoğan as Turkey awaits election results
Ballots for Turkey's presidential and parliamentary elections are pictured at a polling station in Yalova, Turkey June 24, 2018. If a candidate wins just over 50 percent of the vote, he will win the presidency, but if not, there will be a runoff on July 8.
Macron said they agreed on working more with origin and transit countries to prevent migrants boarding boats to Europe, beefing up the EU's Frontex border force and updating the Dublin accords that govern how asylum requests are treated.
Elsewhere, considering following in Mr Salvini's path, Czech Prime Minister Andrej Babis said he was also ready to start turning away migrants if Berlin and Vienna did so, as Germany's interior minister proposed earlier this week.
In a video posted to Facebook on Thursday, Salvini criticised Mission Lifeline for not heeding the instructions of Italian or Libyan authorities.
"We know climate change isn't a matter of faith", she told an worldwide climate meeting in Berlin.
"He wants to show that Italy has been spending more political capital, financial capital, more of its coastguards and defence ressources on this issue than other countries", said Ryan Heath, the political editor of Politico Brussels website. "On the contrary, we would like to send somewhere a few", he added.
"They are in bad conditions, not only medical conditions but (also) psychological conditions, and they really need urgent medical care and psychological care", UNHCR spokesman Marco Rotunno told Reuters.
Arrivals are down some 80 percent this year to around 14,500, as people have turned to other routes. "Finally there is a decision to protect the exterior border", Salvini said.
Grandi, in a statement issued ahead of an European Union summit at the end of next week, said that guarantees were needed so that people in distress at sea are rescued and taken ashore in a predictable way.