• Air strikes target frontlines around Kurdish capital of Irbil • Deployment of US combat troops on ground ruled out • Parallel operation to drop food for civilians on Mount Sinjar

US warplanes bombed Islamic militants outside the Kurdish capital of Irbil on Friday, pulling the US back into Iraq conflict for the first time since President Obama withdrew ground troops in 2011.

After initial strikes outside the city, the US military launched a second and third round later in the day. Then, early on Saturday morning, the US military announced it had conducted a second air drop of food and water for thousands of refugees trapped in Iraq's Sinjar mountains.

The first strikes were limited to the frontlines around Irbil to relieve intense pressure on US-backed Kurdish fighters, but the White House signalled it could expand its military commitment.

That would come only once a more "inclusive" government was formed in Baghdad, giving better representation to Iraq's alienated Sunni minority. However, the Obama administration stressed that sending combat troops back into the country was not an option.

The first 225kg laser-guided bombs were dropped in mid-afternoon by two fighter jets flying from an aircraft carrier in the Gulf. They targeted a howitzer that fighters from the Islamic State (Isis) movement were using to shell Kurdish forces, known as peshmerga.

After a pause of a few hours, a second and third wave of bombing resumed in the evening with attacks on Isis lines outside Irbil, the largest city in Iraq's autonomous Kurdistan region, with a population of 1.5 million inflated by thousands of Arab refugees from fighting elsewhere in Iraq.

The drone hit an Isis mortar position near the city, the Pentagon said, about three hours after Friday's first fighter jet strike. A second pass by the drone killed suspected Isis militants when they "returned to the site moments later".

Over an hour later, four more FA-18 Super Hornets hit an Isis vehicle convoy of seven vehicles while it was parked near the city, as well as a mortar position. Two passes by the jets dropped eight laser-guided bombs.

Together, the three strikes suggested an air campaign to protect Iraqi Kurdistan was beginning to take shape, marking the return of US aircraft to combat in Iraq.

An AP correspondent at the scene reported six bombs dropped at the front at Khazer, the site of a camp for displaced Iraqi Arabs just outside Irbil, abandoned in the face of the Isis offensive. Peshmerga fighters had been told to withdraw from their positions just before the sorties began, but even so, one of the bombs landed within 100 metres of their lines.

Hours earlier, US military transport planes had launched a parallel operation to drop food and water for 40,000 civilians, Kurds mostly of the Yazidi faith, who are besieged on Mount Sinjar, on the western edge of Kurdistan's boundary with the rest of Iraq. The air drops came after reports that children among the stranded population were beginning to die of thirst on the bare, parched mountainside.

The chief spokesman for the Pentagon, Rear Admiral John Kirby, said early on Saturday that the air strikes had been followed up by further relief drops. Three planes delivered 72 bundles of supplies for the refugees. Included were more than 28,000 meals and more than 1,500 gallons of water.

Map of Iraq air strikes

Britain said the RAF would also take part in the mission to airdrop food for the stranded Yazidis. David Cameron said it was the world's duty to help religious minorities "in their hour of desperate need". The prime minister welcomed the American air strikes, but a Downing Street spokeswoman said Britain was "not planning a military intervention".

The White House stressed the limited aims of the operations, aware that one of Obama's proudest achievements in office has been the extrication of America from Iraq after eight gruelling years of war.

A spokesman said that any additional support would be conditional on the formation of an "inclusive" government in Baghdad, a reflection of longstanding American discontent with the prime minister, Nouri al-Maliki, who US and other western officials accuse of pursuing narrow sectarian goals in favour of his fellow Shias, and at the expense of the Sunni minority and national cohesion. One of the US arguments against using force against Isis earlier was that it might relieve pressure on Maliki to open up his government or step down.

The White House pledged that any additional support to a new government would not be prolonged and would not involve ground troops. The US vice- president Joe Biden phoned Iraq's president, Fuad Masum to discuss the strikes and press Baghdad to quickly form a new government, the White House said. "The vice president emphasised the threat Isil presented to all Iraqis and affirmed the US commitment to support Iraq and all of its citizens – from north to south – as they work to defend the country against this international threat," it said in a statement.

Friday's air strikes were framed as being a necessary step to protect a US joint operation centre in Irbil, used to coordinate defences with peshmerga fighters.

"The fact of the matter is we have people in Irbil and if Irbil is allowed to fall, they will be at risk," Ben Rhodes, the national security council spokesman, said.

The president's orders gave his commanders discretion to use air power to protect US military advisers and diplomats in Irbil and Baghdad, and to break the siege of the Yazidis on Sinjar.

It was unclear how long the air strikes around Irbil might last, or how America might extricate itself from an ever evolving and deepening conflict. Isis has proved itself a formidable force, which has rapidly spread its control over a large swath of Syria and Iraq, capturing oilfields and one city after another. Its fighters have declared themselves eager to take on American troops. They have seized control of a dam near Mosul, which if destroyed, could unleash a 20-metre wall of water on the valley, engulfing Iraq's second largest city.

The first US combat operations in Iraq since 2011 began at 1.45pm local time, when the two FA-18 Hornets took off from the USS George HW Bush aircraft carrier in the Gulf.The humanitarian airdrop operations over Mount Sinjar were carried out by giant C17 transport planes flying from an unnamed US base in the region.

On a visit to India, the US defence secretary Chuck Hagel told reporters that more than 60 of the 72 bundles of food and water airdropped on to the mountain had reached the people stranded there.

However, the Yazidis remained encircled by Isis forces, who view their ancient faith as heretical and have executed dozens – possibly hundreds – of Yazidi men during their advance across the region.

Hundreds of Yazidi women have been taken captive by Isis and are being held in schools in Mosul, Kamil Amin, the spokesman for Iraq's human rights ministry said yesterday. He said the ministry learned of the captives from their families. "We think that the terrorists by now consider them slaves and they have vicious plans for them," Amin told The Associated Press. "We think that these women are going to be used in demeaning ways by those terrorists to satisfy their animalistic urges in a way that contradicts all the human and Islamic values."

The International Rescue Committee (IRC) said it was caring for 4,000 Yazidi refugees who had managed to cross the border into Syria. It said the refugees, mostly women and children, were dehydrated, and had survived for up to six days by hiding from Isis militants in the Sinjar mountains.

"Everything humanly possible must be done to prevent further tragedy from occurring on Mount Sinjar. Whether coming overland or through airdrops, there needs to be a coordinated response to ensure aid reaches the 40,000 people stranded on the mountainside," Suzanna Tkalec, IRC's Iraq director, said.

Also on Friday, British Airways announced that it was temporarily suspending flights over Iraq, while Lufthansa and Turkish Airlines suspended their flights into Irbil, which had been an island of calm in Iraq's pervasive violence in the years since the US-led 2003 invasion.

A representative of Iraq's most influential Shia cleric, Grand Ayatollah Ali al-Sistani, called for a more comprehensive international intervention to support the Iraqi government. "The condemnation and consolation statements in support of the affected people, or sending some humanitarian aid, is not enough. Rather, solid plans, in cooperation with the Iraqi government, should be put in place to confront and eliminate the terrorists," said al-Sistani's spokesman Abdul-Mahdi al-Karbalaie in his sermon in the holy city of Karbala.