The summit, the first in Britain since former British Prime Minister Margaret Thatcher welcomed NATO leaders to London in 1990, witnessed the arrival of the heads of state and government of its 28 member-countries.

The Ukrainian crisis, NATO's tasks in Afghanistan, the rise of extremism and sectarian strife in the Middle East and North Africa are expected to top the agenda of the summit.

The alliance will also discuss newer threats such as cyber security and hybrid warfare. Representatives from the United Nations and the European Union will also attend the meeting, together with officials from the International Security Assistance Force (ISAF) in Afghanistan.

The last NATO summit was held in Chicago, US, in 2012.

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