Terrorism deaths fell last year thanks to a weakening of the Islamic State group in Iraq and Boko Haram in Nigeria, but both groups expanded their geographic reach, a new index revealed Wednesday.

Some 29,376 people died from terrorism in 2015, down 3,389 on the previous year and the first fall since 2010, according to the Global Terrorism Index published by the Institute for Economics and Peace.

This fall was thanks largely to 5,556 fewer deaths in Iraq and Nigeria -- a reduction of one third since 2014 -- as military operations weakened IS group and Boko Haram.

However, both groups also expanded their geographical reach last year, with Boko Haram killing more people in Niger, Cameroon and Chad than in the previous year.

IS group affiliates meanwhile carried out attacks in 28 countries in 2015, 15 more than in 2014.

Deaths in OECD countries increased from 77 in 2014 to 577 the following year, more than half of them connected to IS group.

"The attacks by IS militants in Paris, Brussels and in Turkey's capital Ankara were amongst the most devastating in the history of these countries and reflect a disturbing return of the transnational group-based terrorism more associated with al-Qaeda before and immediately after September 11," the report said.

Six countries saw a significantly deteriorating situation in 2015 -- France, Turkey, Saudi Arabia, Kuwait, Tunisia and Burundi.

Deaths from the Taliban in Afghanistan also increased significantly. However, Pakistan, India and Thailand recorded improvements.

More than 90 percent of all terrorist deaths occurred in countries already engaged in some form of internal or international conflict.

And just five countries -- Iraq, Afghanistan, Nigeria, Pakistan and Syria -- accounted for 72 percent of all terrorism-related deaths in 2015.

Meanwhile four groups -- IS group, Boko Haram, the Taliban and al-Qaeda -- were responsible for 74 percent of deaths, with IS group and its affiliates alone killing 6,141 people.

Terrorism cost an estimated $89.6 billion in 2015, down 15 percent on the previous year.