Brussels was hit by a series of explosions on Tuesday, with multiple people reported dead or injured following a suicide attack on its main airport and a bomb on its metro system.
The so-called Islamic State, also known as ISIS, has claimed responsibility, according to the blog of Amaq Agency, which is affiliated to the terrorist group.
The attack follows last week's arrest of Salah Abdeslam in the Belgian capital. Abdeslam is a major suspect in November's Paris attacks that killed 130 people and is known to have links to the so-called Islamic State terrorism group, also called ISIS.
CNBC takes a look at the threat posed by Islamist jihadist groups to countries and regions around the world.
Outside of Belgium, France is considered at high risk of attacks, due to ongoing threats by Islamist groups and recent French military intervention against ISIS in Syria.
A state of emergency was declared following the Paris attacks and remains in place — ISIS has said that France will remain a key target, according to media reports.
Comparatively high numbers of radicalized Muslims have left both Belgium and France to fight for insurgent groups in Iraq and Syria — approximately 37.9 Belgian citizens per million, according to estimates from data provider IHS Jane's.
In the U.K., the threat from terrorism is rated "severe" by its government, meaning an attack is judged highly likely. There are five levels of threat, with "severe" the second-most serious.
- ISIS militants killed 26 regime commandos in Palmyra in Syria on Monday, according to the Syrian Observatory for Human Rights.
Despite recent territorial losses, ISIS still control swathes of eastern Syria. There are continued attacks across Syria, as fighting continues between government forces and insurgent groups that include ISIS. At least 470,000 Syrians have been killed since the war began in 2011, according to the Syrian Center for Policy Research.
On Monday, ISIS killed 26 Syrian government commandos as they battled to retake the UNESCO World Heritage Site of Palmyra, according to the Syrian Observatory for Human Rights.
ISIS and other insurgent groups also control parts of Iraq, particularly in the regions bordering Syria.
The threat from terrorism remains high in Turkey, which neighbors Syria to the north. Further attacks by groups including ISIS and Kurdish extremists are viewed as likely by the U.K. Foreign and Commonwealth Office (FCO).
The U.S. Homeland Security Committee says that an ISIS-linked Syrian individual was planning an attack on the U.S. Consulate in Istanbul in December.
- Three militants opened fire on a beach resort in Cote d'Ivoire this month, killing 18 people. Al-Qaeda in the Islamic Maghreb (AQIM) claimed responsibility for the attack, according to media reports.
- Three police officers and five-alleged Boko Haram suicide bombers were killed during clashes in Niger on Wednesday, Reuters reported.
Al-Qaeda in the Islamic Maghreb (AQIM) has gained a foothold in northern Mali, as well as Mauritania, Niger and Algeria to the north. The jihadist group aligns itself with the main Al Qaeda organization and is dedicated to destabilizing governments in the region and implementing shariah (Islamic) law.
In March, AQIM claimed responsibility, according to media reports, for an attack on a beach resort in Cote d'Ivoire that killed 18 people, including several foreigners.
"There is unquestionably a need to improve border security and intelligence-sharing across West Africa, but ultimately, even the most sophisticated defense apparatus will not be able to prevent such attacks 100 percent of the time," Sean Smith, Africa analyst at risk consultancy Verisk Maplecroft, said in a note after the attack.
Further south, Nigeria and surrounding countries continue to struggle with Boko Haram, an ISIS-aligned jihadist group. It has attempted to establish a "caliphate" or Islamic state in northern Nigeria, exploiting the government's weak control of the large area.
Last Wednesday, Reuters reported that three police officers and five alleged-Boko Haram suicide bombers were killed in Niger during armed clashes.
- At least 50 people were killed by an ISIS truck bomb in western Libya in January.
- The In Salah gas plant in central Algeria was attacked on Friday by explosive munitions.
Several countries in North Africa are at high risk of terrorist attacks, hitting tourism and investment. In 2015, Aon Risk Solutions said Libya was at "severe" risk from terrorism and political violence, with the risk "high" in Algeria and Egypt.
The U.K. FCO has advised against all travel to Libya, due to the ongoing fighting and threat of terrorist attacks and kidnapping of foreigners.
It has also advised that visitors to Algeria avoid the area that borders Libya, as well as the border with Mali, Niger, Tunisia and Mauritania.
On Friday, militants attacked the In Salah gas plant in Algeria. Al Qaeda's North Africa branch claimed responsibility for the attack, which caused no casualties or damage. BP and Statoil, which jointly operate the plant alongside state-owned Sonatrach, said subsequently that they would withdraw staff from two gas plants in Algeria.