A Spanish migrant aid group is deciding whether to continue its Mediterranean rescue mission after it said its boat was targeted by Libya's coastguard.
Proactiva Open Arms, one of three groups still operating off Libya, said its boat was in international waters when it was told to head to port in Libya or come under fire.
Libyan officials did not confirm the incident.
However, the navy last week told NGOs to stay away from the Libyan coast.
Since the EU struck a deal last year with Turkey to seal off the migrant route into Greece, the Libyan route to Italy has become a focal point for migrants and people smugglers. Some 97,000 migrants have reached Italy so far this year.
Libya extended its own search-and-rescue area into international waters last week and a senior naval official accused the aid groups of pretending to rescue migrants trying to make their way across the Mediterranean to Europe.
Tighter restrictions, as well as poor weather, appear to be having an effect on migrant numbers crossing the Mediterranean in recent weeks. Only 2,080 migrants have reached Italy this month, according to UN figures up to 13th August.
Responding to Tuesday's incident, a Libyan navy spokesman told the BBC that organisations that failed to comply with the parameters of the search-and-rescue zone were responsible for their own actions.
Three NGOs, including Save the Children and MSF, have already left the area, saying that threats from the Libyan authorities meant they could not guarantee their safety. Two other groups, SOS Méditerranée and Moas, are still operating in the area as well as the Spanish NGO.
As the Proactiva ship Golfo Azzurro sailed an estimated 24 nautical miles off Libya, a man identified as a Libyan coastguard official told the boat's crew: "You have to sail now towards Tripoli port, you are under custody, sir. If you don't follow the orders we will target you."
The Spanish group's spokeswoman, Laura Lanuza, said it was the second episode in which they had been targeted, after being shot at in the area last week. "Yesterday we were kidnapped and we have to think about the security of our crew so we're on standby," she told the BBC.
"Obviously, this is a result of an orchestrated campaign against NGOs that started last year and different tools have been used to criminalise all of us."
Italy believes the aid boats encourage both migrants trying to cross the sea and the people smugglers who prey on them. Foreign Minister Angelino Alfano has praised Libya's decision to ban NGO boats from its coastal zone.
Italy has helped train the Libyan coastguard in recent weeks. And, backed by the European Commission, it has proposed an 11-point code of conduct for NGOs, warning that those groups that fail to sign it will be barred from Italian ports.
Among the measures are:
- A ban on phone calls to help the departure of migrants
- Allowing an Italian police officer on board to monitor activities
- A requirement for aid boats themselves to take migrants to port rather than transfer them to other ships
- A ban on entering Libyan waters unless there is a clear risk to human life.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.