Jul 11, 2018 17:47 Asia/Tehran

In addition to the delegations of the NATO member states, heads of tens of partner states, as well as the participants of the Resolute Support mission in Afghanistan are invited to attend the summit at different formats.

The summit is shaping up to be one of the most difficult alliance gatherings in years as Trump continues to pressure NATO allies to increase their military spending.

Trump has repeatedly criticized alliance members for their levels of defense spending ahead of the meeting.

In a series of tweets and comments to reporters as he departed Washington, D.C., on July 10, Trump suggested NATO's other 28 members should "reimburse" the United States for its spending on the alliance.

He also repeated misleading statements about which countries are meeting the alliance's Defense Investment Pledge, in which each member state is expected to spend 2 percent of their GDP on defense, and about when those falling short are expected to meet that goal.

Based on current trends, NATO estimates that 15 member states will meet the benchmark by 2024.

NATO headquarters says the alliance is on track to increase its overall defense spending by 3.8 percent in 2018, and a total of eight members are expected to reach the 2 percent benchmark by the end of 2018.

Stoltenberg, for his part, credited Trump at their July 11 breakfast meeting for pushing NATO countries to increase defense spending -- noting that European countries and Canada are expected to "add an extra $266 billion to defense between now and 2024."