Nearly 18,000 people were killed in terrorist attacks in 2013, a 61 percent rise from 2012, resulting in the biggest escalation since records began in 2000, according to a new report.

Eighty-two percent, or 14,722, of all deaths occurred in just five countries: Iraq, Afghanistan, Pakistan, Nigeria and Syria. The main groups responsible? The Taliban, Boko Haram, the Islamic State and al-Qaeda.

All four of these groups used "variations of religious ideologies based on extreme interpretations of Wahhabi Islam," the report said.

The Taliban is responsible for the most deaths, at 8,763, with al-Qaeda next at 8,585. The other two groups, Boko Haram and the Islamic State, haven't been around nearly as long, but together, have killed an excess of 3,000 people in four years, according to the report.

Of the some 10,000 attacks that occurred in 2013 (44 percent more than the previous year), Iraq was the most affected, with more than 6,000 deaths.

The next five countries with the most terrorism-related deaths were India, Somalia, the Philippines, Yemen and Thailand, which had between 1 percent and 2.3 percent of global deaths by terrorism.

The report, titled the Global Terrorism Index, was released by the London-based Institute for Economics and Peace.

"Terrorism doesn't arise on its own; by identifying the factors associated with it, long term policies can be implemented to improve the underlying environment that nurtures terrorism," said Steve Killelea, executive chairman at the Institute for Economics and Peace. "The most significant actions that can be taken are to reduce state-sponsored violence, reduce group grievances and hostilities, and improve effective and community-supported policing."

Killelea told the BBC that the latest increase in terrorism-related deaths is mostly due to the civil war in Syria.

While all four groups hold radical beliefs in Islam, the report noted that there are "many peaceful Muslim-majority countries that do not suffer from terrorism such as Qatar, the UAE, and Kuwait," adding that the causes are often "complex and multidimensional."

The report did, however, find that countries with higher levels of terrorism were found to have the following three "statistically significant factors":

- Greater social hostilities between different ethnic, religious and linguistic groups, lack of intergroup cohesion and high levels of group grievances.

Presence of state sponsored violence such as extrajudicial killings, political terror and gross human rights abuses.

Higher levels of other forms of violence including deaths from organized conflict, likelihood of violent demonstrations, levels of violent crime and perceptions of criminality.