By Michelle Nichols

UNITED NATIONS May 6 (Reuters) - Europe is seeking United Nations Security Council approval to seize boats used to traffic migrants across the Mediterranean from Libya, though diplomats said Russia has signaled it would not allow destruction of the vessels.

European Union leaders agreed last month to "identify, capture and destroy vessels before they are used by traffickers," but it is unclear how that may be achieved and the 28-nation bloc wants U.N. authorization for its operation.

U.N. diplomats, speaking on condition of anonymity, said a resolution has been drafted by European members of the Security Council - Britain, France, Lithuania and Spain - under Chapter 7 of the U.N. Charter which allows the use of force.

The draft text would authorize the EU to intervene on the high seas, in Libyan territorial waters and onshore in Libya to seize vessels "to prevent trafficking, smuggling and illegal migration across the Mediterranean," said a senior U.N. diplomat.

Diplomats said Russia, which has veto power on the Security Council, initially appeared supportive of the measures, but drew the line at approving destruction of boats. The Russian U.N. mission was not immediately available for comment.

EU foreign policy chief Federica Mogherini is due to brief the U.N. Security Council behind closed doors on Monday on the proposed operations. Diplomats said a draft resolution could be circulated to the 15 council members next week.

U.N. Secretary General Ban Ki-moon has said there is no military solution to migrants drowning in the Mediterranean.

About 1,800 migrants have perished during the crossing already this year, the U.N. refugee agency said. Some 51,000 have entered Europe by sea, with 30,500 coming via Italy, fleeing war and poverty in Asia, Africa and the Middle East.

Libya has descended into factional fighting, leaving the country almost lawless nearly four years after the fall of Muammar Gaddafi. Two competing governments backed by militia brigades are scrambling for control of the oil-producing country and the chaos has created havens for Islamist militants.

The group controlling Libya's coastal capital Tripoli said it would "confront" any unilateral EU moves to attack sites used by people-traffickers. Mogherini said any action being considered to stem the flow of migrants should not be perceived as an attack against the Libyan people. (Reporting by Michelle Nichols; Editing by David Gregorio)