Representatives from the 23 countries of the US-led coalition battling the jihadists' self-declared "caliphate" gathered in Rome to assess and renew their efforts.
Italian Foreign Minister Paolo Gentiloni and US Secretary of State John Kerry told their allies that, since their meeting six months ago, the Isis group had suffered setbacks in its core territory in Syria and Iraq.
But the hosts warned that the group is adapting to the pressure on its heartland and is redirecting efforts to Libya, where it has seized new territory, and to attacks like those in Paris, Ankara and San Bernadino, California.
"We are surely not here to brag about anything," Kerry said, after saying Isis fighters have lost 40 percent of their territory in Iraq and 20 percent in Syria.
"We're here to recommit, we're here to re-evaluate, we're here to make judgements about things we have started that we could do better," he said.
Gentiloni said the challenge facing the coalition of mainly Western and Arab nations is stark.
"We know that we have in front of us an organization that is very resilient and able to plan strategically and so we should not underestimate it," he said.
"If anything we need to be ever more wary and more watchful because we know that the more Daesh is squeezed in its core territories the more tempted it is pursue its terrorist activities elsewhere," he warned, using an Arabic name for Isis.
"We are witnessing renewed activity in Libya and in sub-Saharan Africa," he said.
Italy has taken the lead with the coalition in planning for how to address the Isis threat a short boat ride from its southern shores in the Libyan city of Sirte.
"In the past few months the threat in our own homelands has taken on a new more dangerous dimension requiring the coalition's focused and coordinated attention," he added.
'No place to run'
Washington likes to claim that it has built a 66-nation coalition to fight the Isis group - the latest country to join this week was Afghanistan, Kerry said.
But a so-called "small group" of 23 nations has taken the lead in carrying out air strikes in Iraq and Syria and training and arming local forces to fight the jihadists.
Kerry said that the effort now needs to be stepped up, citing the example of the United States' deployment of small numbers of special forces troops inside Syria.
"The challenge now, my friends, is pretty straightforward," he said, in opening remarks to his coalition colleagues ahead of a longer closed-door dialogue.
"To push ahead with a strategy we have learned will work and to do so relentlessly, giving Daesh no time to regroup, no place to run, no safe havens in which to hide.
"This will not be a profitable meeting if we don't talk about what we can do better, because we're still not at the victory that we want to achieve and will achieve."
The nations were to meet for the rest of the day.