LONDON — Britain, Belgium and Denmark lined up Friday behind the United States in its fight against the Islamic State group, agreeing to military operations in Iraq — but drawing a line for now against direct intervention in Syria.

Even the half-step of support, however, offered a boost to President Obama’s effort to cast the fight as a global campaign to beat back a jihadist force that has assembled thousands of radical fighters and seized territory straddling Iraq and Syria.

The entry of the British into the coalition — which now includes five Arab states, France, Australia, the Netherlands, Belgium and Denmark — provides Washington with a broader consensus for what is described as an extended campaign waged without a resolution authorizing the use of military force by the United Nations Security Council.

But Europe’s resolve stopped at the border with Syria, where the Islamic State has built the foundations of its self-declared caliphate. Europeans have been reluctant to take military action inside Syria, in part out of concern about fueling a larger regional conflict, in part because of public opinion in their own nations and in part because of a desire to avoid helping the Syrian government, led by President Bashar al-Assad, to survive the rebellion against him by a wide array of opposition groups, including the Islamic State.