US Secretary of State John Kerry said it was ultimately up to Iraqis to fight back against the self-proclaimed Islamic State group advancing in Iraq despite US-led airstrikes.
"Over time we believe that the strategy will build, the capacity will build, Daesh will become more isolated," he said, referring to the IS by its acronym in Arabic.
"But ultimately it is Iraqis who will have to take back Iraq. It is Iraqis in Anbar who will have to fight for Anbar."
Mr Kerry also said the US was deeply concerned about what he called the "tragedy" in the Syrian town of Kobane, where IS fighters have been tightening their grip.
Kurdish defenders are so far holding off IS militants in the town Kobane, but the fighters struck with deadly bombings in Iraq, killing dozens of Kurds in the north and assassinating a provincial police commander in the east.
The jihadists have laid siege to Kobane for nearly four weeks and fought their way into it in recent days, taking control of almost half of the town.
A UN envoy has said thousands of people could be massacred if Kobane falls.
As night fell, the town centre was under heavy artillery and mortar fire, Ocalan Iso, deputy head of the Kobane defence council, said by Skype from inside the town.
Heavy clashes were under way in the east and southeast, he said, with neither side gaining ground.
Idris Nassan, deputy foreign minister in the Kurdish administration for the Kobane district, said heavy fighting had begun around nightfall in the streets.
Kurdish fighters had caught attackers in an ambush, he said from the town.
After days of IS advances, the Syrian Observatory for Human Rights monitoring group said Kobane's Kurdish defenders had managed to hold their ground.
The Observatory said 36 Islamic State fighters, all foreigners, were killed the previous day, while eight Kurdish fighters had died.
The figures could not be independently verified.
Meanwhile, the top US military officer has suggested that Washington, which has ruled out joining ground combat in either Iraq or Syria, could nevertheless increase its role "advising and assisting" Iraqi troops on the ground in future.
A US-led military coalition has been bombing IS fighters who hold swathes of territory in both Iraq and Syria.
The White House says it will not allow US troops to be dragged into another ground war in Iraq, where President Barack Obama withdrew forces in 2011 after an eight year occupation.
Nevertheless, General Martin Dempsey, suggested in an interview broadcast today that US troops would probably need to play a bigger role alongside Iraqi forces on the ground in future.
"Mosul will likely be the decisive battle in the ground campaign at some point in the future," Gen Dempsey, the chairman of the joint chiefs of staff, told ABC's This Week.
Mosul is the main city in northern Iraq, which IS overran in June and the government has pledged to recapture.
"My instinct at this point is that that will require a different kind of advising and assisting, because of the complexity of that fight," he said.
The fighting in Kobane has taken place within view of Turkish tanks at the frontier, but Turkey has refused to intervene to help defend the city, infuriating its own 15 million-strong Kurdish minority, which rose up in the past week in days of rioting in which 38 people were killed.
Turkish Kurdish leaders have said their government's failure to aid the defence of Kobane could destroy Turkey's own peace process to end decades of insurgency which killed 40,000 people.
Kobane's heavily outgunned Kurdish defenders say they want Turkey to let them bring in reinforcements and weapons to fend off the IS fighters, who seized heavy artillery and tanks seized from the fleeing Iraqi army in June.
In neighbouring Iraq, IS claimed responsibility for a suicide bombing on a security headquarters in a Kurdish-controlled town in the north that killed at least 40 people and wounded 90.
Many of those who died were Kurdish forces veterans who were volunteering to re-enlist.
The three car bombs hit Qara Tapah, which lies close to Jalawla, a key battleground northeast of Baghdad between pro-government forces and IS fighters.
The police chief of Anbar, the mainly Sunni Muslim province that includes the entire Euphrates Valley from the western outskirts of Baghdad to the Syrian border, was killed in a bomb attack on his convoy in an area that had seen clashes between government forces and Islamic State.
The previous day, bombs killed 45 people in Baghdad and its Western outskirts near Anbar.
The US used army Apache attack helicopters for the first time this past week to provide close air support to Iraqi forces in Anbar west of Baghdad.
Use of low-flying helicopters is far riskier than bombing from jets but allows closer cooperation with troops engaged in combat on the ground.
Gen Dempsey said the decision was taken to halt fighters who might otherwise have been able to attack Baghdad's airport, which is on the capital's western outskirts.
