MAIDUGURI, NIGERIA - Millions of children in northeast Nigeria, where Boko Haram is waging an insurgency, have been vaccinated against measles in recent weeks but many others could not be reached due to the threat posed by the Islamist militants, the United Nations said on Thursday.

Some 4.7 million children in Adamawa, Borno and Yobe – the three states hit hardest by Boko Haram violence - were given vaccines in the past two weeks to protect them against the highly contagious virus, the U.N. children's agency (UNICEF) said.

"Security has improved in some areas so we have acted quickly to access places we could not previously reach and protect children from the spread of a very dangerous disease", UNICEF representative Mohamed Fall said in a statement. "We are still extremely concerned about children living in large areas of Borno state that are not yet accessible."

Boko Haram has killed about 15,000 people and forced more than two million to flee their homes in a seven-year-long campaign to create an Islamic caliphate in northeast Nigeria.

While the Nigerian army has recaptured large swathes of land from the jihadist group, much of the northeast is still cut off from humanitarian aid with Boko Haram fighters prowling in the bush and regularly carrying out attacks and suicide bombings.

Around two thirds of Borno's health facilities have been damaged or destroyed during the insurgency, disrupting health care for millions of people, the World Health Organization said.

Yet UNICEF said it had worked with the government to repair health centers and set up temporary clinics to provide more than four million people with services ranging from antenatal care to treatment for malaria, diarrhoea, pneumonia and measles.

Some 25,000 children in Nigeria suffered measles last year, at least 100 of whom died, according to the U.N. agency.

Measles, which spreads through direct contact and the air, is one of the biggest killers of children worldwide, but can be prevented with two doses of a widely available vaccine.

The number of deaths from measles has fallen by 79 percent worldwide since 2000, due mainly to mass vaccination campaigns, but nearly 400 children still die from the disease every day.

The United Nations in December doubled its humanitarian funding appeal for northeast Nigeria to $1 billion for 2017 in a bid to reach some seven million people hit by the Boko Haram insurgency who need life-saving help.

Last year's $484 million appeal has only been 53 percent funded, the U.N.'s Financial Tracking Service shows.