NEWPORT, Britain, Sept. 4 -- The North Atlantic Treaty Organization (NATO) on Thursday kicked off its 26th summit at the Celtic Manor Resort in Newport, southeast Wales of Britain, with nearly 60 world leaders invited to attend the two-day gathering.

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

On his arrival at the summit, NATO Secretary General Anders Fogh Rasmussen said NATO is facing a "dramatically changed security environment" and the gathering will take important steps to counter threats and strengthen the defense of its allies.

"Ultimately by working together we are stronger, whether in standing up to Russia or confronting ISIL (the Islamic State of Iraq and the Levant). So in Newport today we must summon up the shared resolve that inspired NATO's founding fathers," British Prime Minister David Cameron and U.S. President Barack Obama wrote in a joint article published Thursday in British newspaper The Times.

"The truth is that today NATO is as vital to our future as it has ever been in our past," they said.

The summit took place in the context of worsening global security environment and growing instability in Eastern Europe, the Middle East and North Africa.

The Ukraine crisis, NATO's tasks in Afghanistan, and 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, missile defense and hybrid warfare.

Formal sessions of the summit will start with a meeting on Afghanistan, as NATO is completing its combat mission at the end of 2014 and readjusting its partnership with Kabul.

High-level representatives from the United Nations and the European Union (EU) will also attend the meeting, together with leaders from International Security Assistance Force (ISAF) nations, Afghanistan and Japan.

A meeting of the NATO-Ukraine Commission, which aims to bolster NATO-Ukraine cooperation, will also be held on Thursday with Ukrainian President Petro Poroshenko being invited to attend.

On Friday, NATO leaders will discuss NATO's ability to respond to current security risks in regions to the east and south of the alliance and future challenges.

They will also examine issues of defense investment, capacity building, training and modern equipment.

Besides, the bloc's defense and foreign ministers will hold parallel sessions to talk with four countries that aspire to join NATO, and meet with other international organizations to discuss security challenges.

Senior representatives from more than 30 partner countries and international organizations like the United Nations, the Organization for Security and Cooperation in Europe and the EU will also be present at the meetings.