The fight against jihadist group Islamic State (IS) will take years, a US military spokesman has told the BBC.
Rear Admiral John Kirby also said that US-led air strikes against IS in Syria had disrupted the group’s capabilities.
The remark came as President Barack Obama thanked Arab states for help and Secretary of State John Kerry said more than 50 nations had agreed to fight IS.
IS has seized large areas of Syria and Iraq, and the US has launched nearly 200 air strikes in Iraq since August.
Monday’s strikes however expanded the anti-IS campaign across the border into Syria for the first time.
Activists say at least 70 IS militants and 50 other al-Qaeda-linked fighters were killed in the strikes.
‘No safe havens’
Speaking in Washington, Rear Adm John Kirby said the air strikes in Syria had successfully degraded IS’s capabilities.
“We think we have hit what we were aiming at,” he said.
However, IS was good at adapting and reacting to changes, he said, adding that the group presented a “serious threat” that would not be eliminated “within days or months.”
“It’s going to take a serious effort by all involved. We do believe that we’re talking about years here.”
Meanwhile, Mr Kerry told reporters that more than 50 countries had agreed to join efforts to fight IS.
“We will not allow these terrorists to find a safe haven anywhere,” he said.
On Tuesday, Mr Obama hailed the support of Arab nations in the air strikes, saying: “This is not America’s fight alone.”
Saudi Arabia, the United Arab Emirates, Jordan, Bahrain and Qatar had taken part in or supported the strikes in Syria, Mr Obama said.
The Pentagon said warplanes, drones and Tomahawk cruise missiles were used in the strikes.
The strikes targeted the IS main headquarters in its stronghold of Raqqa, north-eastern Syria, as well as training compounds, vehicles and storage facilities in several other areas.
They were organised in three separate waves with US fighter jets carrying out the first set, and Arab nations participating in the second and third, US military officials said.
US state department spokeswoman Jan Psaki said the US had warned Syria in advance “not to engage US aircraft”.
But she added that Washington had not requested permission or given advance notice of the timing of the attacks.’Imminent attacks’
President Obama said al-Qaeda-linked militants, known as the Khorasan Group, were also targeted by air strikes in Syria.
US officials say the group had been plotting “imminent attacks” against the West, and had established a safe haven west of Aleppo.
As well as informing Syria’s government of the impending strikes, the US reportedly told Iranian officials attacks were imminent, Reuters reports.
Syrian President Bashar al-Assad, quoted by state media, said he supported any international efforts to combat “terrorism” in Syria.
However, Iranian President Hassan Rouhani, a Syrian government ally, said military action in Syria lacked “legal standing” without a UN mandate or approval from the Syrian government.