NAIROBI, June 26 (Reuters) - Kenya aims to reduce by almost half the population of Dadaab refugee camp which is home to about 326,000 mostly Somali refugees by the end of the year, a committee that groups Kenya, Somalia and the U.N. refugee agency UNHCR said.

Kenya has said it wants to close the camp, which was once home to more than half a million refugees, citing security threats. Nairobi says the Somali Islamist group al Shabaab has used the camp as a recruiting ground to launch attacks on Kenya.

But Kenya has been urged by the United States, the United Nations and others to ensure no one is forced to return to Somalia, which is still struggling to rebuild after decades of conflict and continues to face an Islamist insurgency.

After a meeting on Saturday, the so-called Tripartite Commission said Dadaab had 326,000 refugees at the end of May, already 100,000 fewer than five years earlier, many of whom it said were believed to have returned to Somalia.

"The parties noted the prospect of the reduction of the population in the Dadaab camps by 150,000 individuals by the end of 2016," the joint communique said, referring to string of sites that make up the Dadaab camp complex.

It said the number would be reduced due to "voluntary returns to Somalia, relocation of non-Somali refugees, the de-registration of Kenyan citizens who registered as refugees, and a population verification exercise."

The U.N. had said earlier this year it planned to reduce the number of people in Dadaab by 50,000 by the end of 2016, but had said that could be a challenge given continued security concerns in Somalia and lack of schools and other public services there.

A U.N. official said on Sunday that the new target of reducing the Dadaab population by 150,000 was a prospective figure and it was not guaranteed to be achieved.

The Tripartite Commission also said it had agreed to meet again in October 2016 to review progress made "on the voluntary repatriation of Somali refugees from Kenya."

(Writing by Edmund Blair; Editing by Raissa Kasolowsky)