United States president Barack Obama and French president Francois Hollande have held talks at the White House, with the Paris attacks dominating their agenda.
It comes as France seeks an increased commitment from the United States and others in combating the threat of I-S.
France and the United States have agreed to increase their airstrikes in Syria and Iraq targeting I-S after the deadly attacks in Paris.
Barack Obama has emphasised the long friendship between the two countries and pledged to step up efforts against I-S, also known as ISIL or Daesh, with its European partners.
"(The) terrorist group ISIL, or Daesh, and its murderous ideology pose a serious threat to all of us. It cannot be tolerated. It must be destroyed, and we must do it together. This is the unity of purpose that brings us here today."
Mr Hollande says France is maintaining its position that it will not put troops on the ground in Syria to fight IS.
"France will not intervene militarily on the ground. It is for the local sources to do so. We've been supporting them for a number of months. We will continue to do so. And they will do the job on the ground after our strikes that will enable them to do so."
The two leaders have agreed on the importance of closing the Turkish border to limit the movement of militants into Europe.
Mr Hollande says it is also about stopping the infiltration in other ways.
"It is about destroying Daesh, no matter where they are. It is about taking out their financing, hunting down their leaders, dismantling their networks and taking back the land they currently control."
Mr Obama says national security is an absolute priority, and he has pledged US assistance in tackling the European humanitarian crisis.
"Here in the United States, refugees coming to America go through up to two years of intense security checks, including biometric screening. Nobody who sets foot in America goes through more screening than refugees. And we're prepared to share these tools with France and our European partners."
The two leaders met following attacks in Paris that killed 130 people at the national sports stadium, a concert venue and bars and restaurants in the heart of the French capital.
IS has claimed responsibility for the attacks.
But it is not only those attacks spurring the talks to defeat IS.
The group has also claimed responsibility in recent weeks for the bomb that brought down a Russian passenger plane over Egypt, killing all on board.
And now it is claiming responsibility for a bomb attack on a presidential guard bus in the Tunisian capital Tunis that killed at least 12 people.
As well, it is claiming responsibility for two suicide bombers and a gunmen who killed four people at a hotel in Egypt's North Sinai.
French interior minister Bernard Cazeneuve has met with the head of the French Muslim Council to discuss the consequences of the Paris attacks on France's Muslim community.
Mr Cazeneuve says France will not tolerate hate of any kind and those preaching hatred will be forced to leave the country.
"This firmness leads me to carry out the expulsion of those foreigners on French soil who engage in, and spread, this message of hate. I am doing this for the French people, to protect the Muslims here in France who are hurt by the corruption of their religion and who are also amongst the victims of such barbaric acts."
The president of the French Council of the Muslim Faith, Anouar Kbibech, says plans are place for tighter controls on the formal training of imams.
He says it is an attempt to fight what he calls radicalisation present in some mosques.
"Furthermore, we are also in the process of devising an Imam Charter, where each imam will commit to a certain number of important issues -- those of speech, vision -- so that they can propagate and spread a free and tolerant Islam, an Islam which finds the right balance."
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