ROME (Reuters) - The U.N. refugee agency is seeking to open a refugee transit center in Tripoli early next year to resettle or evacuate as many as 5,000 of the most vulnerable refugees out of Libya each year, a senior U.N. official said on Friday. It is a small fraction of the total number of Libya s migrant population, estimated at as many as 1 million, but would be a welcome outlet for the 43,000 refugees that the United Nations High Commissioner for Refugees (UNHCR) estimates are now trapped in Libya.  We hope to have the (written) authorization soon,  Roberto Mignone, the UNHCR s representative in Libya, told Reuters in Rome. The U.N.-backed Tripoli government has already approved the project verbally, he said. The center, where migrants will be able to come and go as they please, will be in a former immigration police training facility. Once refurbished, it will be able to temporarily accommodate as many as 1,000 refugees and could be running by early 2018, Mignone said. Italy has become the main migrant route to Europe since an agreement between the EU and Turkey shut down smuggling through Greece last year, but arrivals have fallen sharply since July, when an armed group clamped down on departures. With the backing of the European Union, Italy has financed, trained and equipped by the Tripoli-based coast guard. With a national election due early next year, Italy is also promising tens of millions of euros to Prime Minister Fayez al-Seraj and municipal governments to put a stop to smuggling.  This strategy has drawn criticism from humanitarian groups that point to the dire conditions inside state detention centers, where Mignone said some 6,000 now are held, and in the much more numerous  camps  where smugglers hold migrants, often extorting them or forcing them to labor for free. Italian Interior Minister Marco Minniti has said he is depending on the U.N. refugee and migration agencies to improve conditions for refugees and migrants now trapped in Libya.  We can t be the only solution,  Mignone said, also because Libya remains very dangerous and international staff still have very limited access to the country.  There are still many risks  for international staff, he said.  A month-and-a-half ago, there was an attack on a U.N. convoy 30 km (19 miles) from Tripoli with bazookas and machine guns. It was a miracle that the worst was avoided.  Some 1,200 of the most vulnerable refugees   which includes women, children, the sick or disabled and the elderly   have already been released from detention centers at the request of the UNHCR, and about 800 more should be let out soon, Mignone said.  While the UNHCR hopes to resettle many of them, it is a lengthy process. Many countries do not have a permanent diplomatic presence in Tripoli, further complicating matters. So the agency will seek to evacuate most of them, Mignone said, to emergency transit centers in Romania, Slovakia or even Costa Rica, where they will have more time to apply for resettlement. The agency is currently working to open another emergency transit center in Niger, he said. 