The summit is taking place at a crucial time, he said, adding that the alliance is faced with a dramatically changed security environment.
"To the east, Russia is attacking Ukraine," Rasmussen said. To the southeast, we see the rise of a terrorist organization -- the so-called Islamic State -- that has committed horrific atrocities. To the south, we see violence, insecurity, instability. Here at the summit we will take important steps to counter these threats and to strengthen the defense of our allies."
Readiness action plan
The secretary general said that during the summit he expects NATO member states will agree to a readiness action plan aimed at speeding NATO's response in defense of its allies.
"On defense investment, we will turn the corner and reverse the trend of declining defense budgets," he said.
Also planned are discussions on what individual allies and NATO can do to counter the threat from the Islamic State of Iraq and the Levant.
NATO hasn't yet received a request from the Iraqi government for assistance in battling ISIL, the secretary general said. "In that respect let me remind you that NATO has assisted Iraq in the past," he added. "We had a training mission in Iraq until 2011, and if the Iraqi government were to request resumption of such training activities I think NATO allies would consider such a request seriously."
Rasmussen said NATO member states also will take steps to enhance cooperation with Ukraine and other partner nations. NATO welcomes all efforts to find a peaceful solution to the crisis there, he noted.
"Having said that, I also have to say that what counts is what is actually happening on the ground," he said. "And we are still witnessing, unfortunately, Russian involvement in destabilizing the situation in eastern Ukraine. So we continue to call on Russia to pull back its troops from Ukrainian borders, stop the flow of weapons and fighters into Ukraine, stop the support for armed militants in Ukraine and engage in a constructive political process. That would be a genuine effort to facilitate a peaceful solution to the crisis in Ukraine."
Ukrainian President Petro Poroshenko will meet with NATO leaders today to adopt a joint declaration that will outline concrete steps to enhance the NATO-Ukraine partnership, Rasmussen said.
NATO members also will discuss what the future relationship with Afghanistan will look like after the International Security Assistance Force mission ends this year, he added.
"This summit will shape future NATO," Rasmussen said. "It will demonstrate our resolve, our unity, our solidarity.
"Surrounded by an arc of crises," he continued, "our alliance, our transatlantic community, represents an island of security, stability and prosperity. And here at the summit we will strengthen our transatlantic bond as the bedrock of security in Europe and North America."
Copyright 2015 Nexstar Broadcasting, Inc. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.