* NATO will use force if needed, head of operation says
* Embargo to block arms, mercenaries from entering Libya
* 7 countries have joined operation so far
By Philip Pullella
NAPLES, March 24 (Reuters) - NATO will enforce the arms and mercenaries blockade on Libya "robustly," with force if needed, and hopes more nations will join those who have committed ships, planes and submarines, the operation head said on Thursday.
"We will ensure the free flow of legitimate shipping which the people of Libya need," Vice Admiral Rinaldo Veri, head of Operation Unified Protector, told a news conference at NATO's Southern Europe headquarters in Naples.
Veri said his force had begun operations on Wednesday evening with ships already in the area, but had not detected any attempt to breach the embargo so far.
He said his force was still "in a build-up phase" and would have enough assets in a few days to fully sustain the embargo.
He said Italy, Britain, Greece, the United States, Canada, Spain and Turkey had already contributed or committed ships or other military assets and that he hoped more would join.
In response to a question, the admiral would not be drawn into speculating if France will eventually join the mission.
"The force generation process is still under way. It is still in the build-up phase." He added: "I confide that we will have enough assets to carry out our mission."
Veri noted that U.N. Security Council Resolution 1973 authorised "any means" to prevent arms or mercenaries from entering Libya. If forces from participating nations encountered any resistance, "the use of force may be necessary," he said.
"My task group commanders are empowered to enforce the embargo robustly all the way up to a full boarding."
Veri said the coalition would be using ships, submarines and maritime patrol aircraft to enforce the embargo. The force would be patrolling a wide maritime area "but our focus is mainly on what is going in and coming out of Libya".
He acknowledged that NATO could not hermetically seal off Libya or succeed in fully blocking arms or mercenaries.
"The maritime route is the easiest way of getting arms into Libya. What we are doing now is cutting off that area. I know we cannot close all the windows but one thing for sure is that we are closing the main front door," he said.
While the fighting raged, NATO again failed to agree to take over command of the military operation "Odyssey Dawn" from the United States, chiefly because of objections from Turkey, diplomats said.
The United States, with its forces already tied up in Iraq and Afghanistan, said it wants to give up its lead role in Libya in a "matter of days" and NATO to play an important role in the command of the operation, although the exact structure of its role was still under discussion.
France, which launched the air campaign against Libya with Britain and the United States on Saturday, argues that having the U.S.-led NATO in charge would erode Arab support because of the alliance's unpopularity in the Arab world.