President Obama says "more than 40 countries" have offered to help in the fight against Islamic State terrorists.
He stressed that the US would not lead the fight alone, but would "do things differently" by building a strong international effort against the terror group, who are also known as Isis or Isil.
He told US military personnel: "We will lead a broad coalition of countries who have a stake in this fight because this is not simply America vs Isil, this is the people of the region fighting against Isil."
"It is the world rejecting the brutality of Isil and fighting for a better future for our children and our childrens' children - all of them," he added.
President Obama has made clear that the US will not send ground troops to fight Islamic State terrorists in Iraq.
Speaking at US Central Command in Florida, Mr Obama told military personnel: "As Commander-in-Chief I will not commit you and the rest of our forces to fighting another ground war in Iraq."
He said US personnel in Iraq were there to "support Iraqi forces on the ground as they fight for their own country against these terrorists".
One of the most senior US military officials has said he may yet advise President Obama to send ground troops into Iraq to combat Islamic State militants.
General Martin Dempsey said that if there were "threats to the United States" he would make a recommendation that "may include the use of US military ground forces".
ITV News Washington Correspondent Robert Moore reports:
The US Defence Department are in complete agreement that the United States and its allies must take action against the Islamic State, Defence Secretary Chuck Hagel told a hearing in Capitol Hill.
American military power alone cannot eradicate the threats posed by ISIL to the United States, our allies, and our friends and partners in the region.
ISIL poses a real threat to all countries in the Middle East, our European allies, and to America.
More than 40 nations have already expressed their willingness to participate in this effort, and more than 30 nations have indicated their readiness to offer military support. President Obama, Vice President Biden, Secretary Kerry and I and others have been working in recent weeks to unite and expand this coalition.
Islamic State fighters have shot down a Syrian warplane using anti-aircraft guns, the first time the group has downed a military jet since declaring its cross-border caliphate in June, a group monitoring the civil war has said.
The plane came down outside Islamic State's stronghold of Raqqa city, 250 miles northeast of Damascus, during air strikes on territory controlled by the group, a resident said.
The Syrian Observatory for Human Rights, a monitoring group which gathers information from a network of activists on the ground, reported five air raids on Raqqa on Tuesday.
Rami Abdulrahman, who runs the organisation, cited sources close to Islamic State as saying the plane had been shot down.
The speaker of the Syrian Parliament has reportedly urged US Congressional leaders not to arm Syrian rebels.
NBC News correspondent Bill Neely said that Jihad al-Laham told Nancy Pelosi and John Boehner Syrian rebels had "handed US journalists" to Islamic State militants.
Speaker of #Syrian Parliament writes to US Congressional leaders asking not to arm Syrian rebels,says it's against UN SecCouncil resolutions
Airstrikes on Islamic State targets must distinguish between military and civilian objects, and be proportionate, UN investigators have said.
According to Reuters, the UN investigators called on countries preparing military strikes against IS to abide by the laws of war.
An Iraqi civilian who was found dead four months after being released by British armed forces during the Iraq war did not have his human rights breached, judges in the European Court of Human Rights have ruled.
US Defence Secretary Chuck Hagel and chairman of the joint chiefs of staff Army General Martin Dempsey have been scheduled to testify before Congress over President Obama's strategy to combat Islamic State extremists in Iraq and Syria, according to the Associated Press.
The hearings are the first in a series that will measure the president's ability to rally congressional support.
President Obama announced last week an intensified phase in the US counter-offensive against IS in Iraq, which could entail strikes inside Syria. The president said he had the authority to order the airstrikes without new congressional approval.
US military forces conducted two airstrikes near Sinjar and south west of Baghdad, destroying six vehicles belonging to Islamic State militants, US Central Command has said.
These strikes were conducted under authority to protect US personnel and facilities, support humanitarian efforts, and help Iraqi forces on the offensive against ISIL terrorists.
US Central Command has conducted a total of 162 airstrikes across Iraq.