* PM aims to retake big city from Islamic State by year-end
* Most of the 560 new troops to work out of Qayara air base
* It is second such force increase in less than three months
* US will help turn damaged air base into logistics hub 

The United States is stepping up its military campaign against Islamic State (IS) by sending hundreds more troops to assist Iraqi forces in an expected push on Mosul, the militants' largest stronghold, later this year.
US Defense Secretary Ash Carter made the announcement on Monday during a visit to Baghdad where he met US commanders as well as Iraqi Prime Minister Haider al-Abadi and Defence Minister Khaled al-Obeidi.
Most of the 560 troop reinforcements will work out of Qayara air base, which Iraqi forces recaptured from Islamic State and plan to use as a staging ground for an offensive to retake Mosul, Iraq's second biggest city.
Government forces said on Saturday they had recovered the air base, about 60 km (40 miles) from the northern city, with air support from the US-led military coalition.
"With these additional US forces I'm describing today, we'll bring unique capability to the campaign and provide critical support to the Iraqi forces at a key moment in the fight," Carter told a gathering of US troops in Baghdad.
The latest force increase came less than three months after Washington announced it would dispatch about 200 more soldiers to accompany Iraqi troops advancing towards Mosul.
Carter told reporters ahead of Monday's trip that the United States would now help turn Qayara into a logistics hub.
The airfield is "one of the hubs from which ... Iraqi security forces, accompanied and advised by us as needed, will complete the southernmost envelopment of Mosul," he said.
The recapture of Mosul, Islamic State's de facto Iraqi capital from which its leader declared a modern-day caliphate in 2014, would be a major boost for Abadi and US plans to weaken IS, which has staged attacks in the West and inspired others.
Two years since Islamic State seized wide swathes of Iraq and neighbouring Syria in a lightning offensive, the tide has begun to turn as an array of forces lined up against the jihadists have made inroads into their once sprawling territory.
IS has increasingly resorted to ad hoc attacks including a bombing in the Iraqi capital last week that left nearly 300 people dead - the most lethal bombing of its kind since the 2003 US-led invasion that ousted Saddam Hussein.
US and Iraqi officials have touted such bombings as proof that battlefield setbacks are weakening Islamic State, but critics say a global uptick in suicide attacks attributed to the group suggests the opposite.
"In fact, it demonstrates (Islamic State's) strength and long-term survival skills," terrorism expert Hassan Hassan wrote in a recent article. "The threat is not going away."

Repairs needed

A senior US defence official said Qayara air base would be "an important location for our advisers, for our fire support, working closely with the Iraqis and being closer to the fight."
Carter compared its strategic importance to that of a base near Makhmour, a hub for Iraqi forces on the opposite side of the Tigris river that is also used by US troops. A US Marine was killed in Makhmour in March when it was shelled by IS.
US forces had already visited Qayara to check on its condition and advisers can offer specialised engineering support in Mosul, where Islamic State has blown up bridges across the Tigris, US officials said.
Iraqi forces were already improving the base's perimeter in case of a counterattack from the nearby town of Qayara which IS still holds, another US official in Baghdad said.
Islamic State has suffered a number of territorial losses in recent months including the Syrian town of al-Shadadi, taken by US-backed Syrian forces in February, and the Iraqi recapture of Ramadi in December and Falluja last month.
Abadi has pledged to retake Mosul by the end of the year.