Iraq’s prime minister is leaving for Washington for talks with US President Barrack Obama, whom he is expected to ask for substantial weapons deliveries.
Haider al-Abadi will also seek to agree a strategy for dislodging Islamic State (IS) militants from the large parts of the country which they still control.
Last week, Mr Abadi launched a campaign to drive them out of Iraq’s biggest province, Anbar, west of Baghdad.
IS responded with a renewed onslaught on the provincial capital, Ramadi.
The US-led coalition carried out air strikes on IS positions on Sunday in an attempt to halt the assault, inflicting heavy damage on large military units, officials said.
Tanks and other armoured vehicles were also hit near the northern IS-held city of Mosul and Iraq’s main oil refinery at Baiji, which is also under attack.
The BBC’s Jim Muir in Beirut says the battle on the outskirts of Ramadi is the opening phase of what is likely to be a very long and hard struggle by Iraqi government forces to regain Anbar province.
Elsewhere, it seems to be IS which is on the offensive, our correspondent adds.
In one area further north of Ramadi, militants are reported to have surrounded an Iraqi army unit and attacked its base with suicide bombers. They are also said to be massing to the south-east of the city.
Mr Abadi’s visit to Washington is his first since he took over from Nouri al-Maliki after elections last year.
The prime minister will seek Mr Obama’s help to acquire billions of dollars of advanced US weaponry, including unarmed surveillance drones, Apache attack helicopters and ammunition. He reportedly wants to defer payments for the purchases.
Our correspondent says Mr Abadi will also be hoping for an understanding on how to pursue the battle against IS and how US involvement – through coalition air strikes and military advice on the ground – can be harmonised with Iran’s role, exercised notably through the Shia militia it supports.
The recent battle to recapture Tikrit showed that he needs both, our correspondent adds.
Shia militia played a key role on the ground, but they could not make the final push without the help of coalition air strikes.
In Anbar province, the situation is even more difficult, our correspondent says.
The use of Shia militia in such a heavily-Sunni heartland is rejected by many Sunnis.
Sunni tribes played a major role in ousting al-Qaeda in Iraq – a precursor to Islamic State – from Anbar eight years ago, but the latter felt betrayed and abandoned by the Shia-dominated government in Baghdad.
They will take a lot of convincing and reassurance before putting their fate in the government’s hands again, our correspondent adds.
On Sunday, the US ambassador in Baghdad warned that the coalition would not be able to conduct air strikes in support of Iraqi government forces in Anbar if Shia militia were fighting alongside them, Iraqi officials told the New York Times.
When militia withdrew from the Ramadi area, the coalition immediately responded by stepping up bombing raids, the officials said, although a spokesman for the ambassador denied that there had been any conditions.