The Brussels attacks are a stark reminder that European countries need to step up their efforts against the Islamic State group, US Defense Secretary Ashton Carter said.
The United States has been pressing its European and Arab allies to contribute more to the US-led campaign against the group in Iraq and Syria.
The IS group has claimed responsibility for Tuesday's suicide bombings on the Belgian capital's airport and metro that killed 31 people and wounded another 300.
"The thing that I think the Brussels event is going to further signify to Europeans is that they -- as we have been accelerating our campaign to defeat ISIL in Syria, in Iraq and elsewhere -- they need to accelerate their efforts and join us," Carter told CNN on Wednesday.
"But (for) anyone doubting in Europe, it's a reminder that we have to accelerate our effort."
Washington has boosted its own special forces presence in Iraq and Syria, and is now letting its military advisers move closer to the frontlines.
A US Marine manning a newly established US artillery position in northern Iraq was killed over the weekend in a rocket attack, underscoring the risks.
Carter said he expected Mosul, a major IS bastion in northern Iraq, would be retaken by Iraqi troops within a year.
"If the Iraqi government continues to support their own armed forces and we continue to support them, there is no reason we have to wait a year for the collapse of Mosul," Carter said.
"But I'm not prepared to give you a timetable either. This is a war."
"I think we need to do and are doing everything we can to accelerate that schedule," he added, predicting that: "We'll be doing more."
The Iraqi forces, as they advance on Mosul will need air support, as well as training and equipment, Carter said.
"They need things like bridges to cross the Tigris River. They need lots of things," he added.
Iraq, meanwhile, announced Thursday it had launched the first phase of operations to retake Mosul.
It's in Mosul, Iraq's second largest city, that IS declared its Islamic "caliphate" in 2014.