The EU is set to launch a naval mission on Monday against gangs smuggling migrants from Libya but it will be limited to intelligence-gathering for now because of a lack of UN authority or Libyan consent.

The operation is part of a stepped-up European response to a wave of thousands of people from Africa and the Middle East making the dangerous crossing from Libya to Europe.

Many have drowned in the Mediterranean, including around 800 killed in a shipwreck in April.

EU foreign ministers meeting in Luxembourg on Monday are expected to agree to launch the mission, diplomats said, but will authorise only a first phase using ships and planes to gather intelligence about the migrant traffickers' activities in international waters.

A second phase envisages boarding ships, arresting smugglers and disabling boats on the high seas, while a third could involve similar operations in Libyan waters or special forces missions on land against smugglers' boats.

EU governments will not agree to move beyond the intelligence-gathering phase without Libyan authorities' consent to the mission and a UN Security Council resolution – neither of which has so far been obtained.

"Ministers would only be launching phase one, which is essentially intelligence-gathering on the high seas. They would only have a legal mandate to launch phase one in the absence of a Security Council resolution," one EU diplomat said.

Winning Libyan consent is difficult because the country is in turmoil, with two rival governments and parliaments fighting for control.

European members of the 15-member Security Council were drafting a resolution to approve the operation under Chapter 7 of the UN Charter, which allows the use of force.

Securing Libyan consent and a UN resolution could be tied to a successful outcome to UN envoy Bernardino Leon's so far fruitless attempts to persuade Libya's rival factions to agree to form a national unity government.

"What (the Libyans) are basically saying now is that 'We're not going to decide (on consent for the EU mission) until we know where we are with the Leon process,'" a senior Security Council diplomat said in New York.

"It is stuck, frankly," said the Brussels-based diplomat.

Some EU diplomats, concerned about mission creep, want a firm commitment that any decision to move to phase two or three would require separate approval by EU governments.

Despite the obstacles, officials were meeting in Brussels on Thursday to pledge equipment and personnel for the mission.

John Dalhuisen, of rights group Amnesty International, said there was very little the EU flotilla could do without access to Libyan territorial waters.

"You will have a bunch of boats ... sitting about 30 nautical miles (56 km) off Libya's territorial waters able in practice to serve no other useful function than search and rescue," he told a news conference.