By Kieran Guilbert

DAKAR (Thomson Reuters Foundation) - A spate of attacks by Boko Haram in southeastern Niger in recent months is hindering the delivery of aid to more than 200,000 people forced from their homes, aid agencies said on Thursday.

Niger's Diffa region is hosting around 220,000 displaced people - split almost evenly between uprooted Nigeriens and Nigerian refugees - who have fled violence by the Islamist militants on both sides of the border, the United Nations says.

The region has been targeted around 15 times since September in attacks blamed on Boko Haram, causing thousands more to flee and restricting access to those in need of aid, said the U.N. Office for the Coordination of Humanitarian Affairs (OCHA).

Boko Haram militants have killed about 15,000 people and displaced some 2.6 million in Cameroon, Chad, Niger and Nigeria during a seven-year campaign to carve out an Islamist caliphate.

The Islamist group still launches deadly attacks despite having been driven out of much of the territory it held in 2014.

"The humanitarian situation is really dire, and deteriorating," Geoffrey Denye, a spokesman for aid group World Vision, told the Thomson Reuters Foundation by phone from Chad.

"The displaced are getting harder to reach and humanitarians are required to take more risks to provide aid due to the insecurity."

Several aid agencies said the military had restricted humanitarian access to areas around the town of Bosso, where 32 soldiers were killed in June by Boko Haram in the militant group' deadliest attack in Niger since April 2015.

Along with Chad, Cameroon, Nigeria and Benin, Niger has contributed troops to a 9,000-strong regional task force dedicated to fighting the group.

Among the recent attacks in Diffa, supplies and essential medicines have been looted from health facilities. This could scare local health workers and deter from working in the region, according to medical charity Medecins Sans Frontieres (MSF).

"On the whole, the aid response is not as effective as it should be," said Mari Carmen Viñoles, Niger program manager for MSF. "There is a problem around coordination, the different responsibilities of various humanitarian actors are not clear."

Some of the displaced are in refugee camps, others live in makeshift huts along Niger's main highway, but most are dotted across than 100 villages and informal sites.

"These people are spread across an area the size of Belgium, and most are without livelihoods or access to social services," said U.N. refugee agency (UNHCR) spokesman Benoit Moreno.

"It's a challenge for humanitarian organisations," he added.

(Reporting by Kieran Guilbert, Editing by Katie Nguyen. Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women's rights, trafficking, corruption and climate change. Visit news.trust.org)