RAF Tornado jets in Cyprus for Iraq aid mission
RAF Tornado jets have arrived in Cyprus from the UK and are set to support efforts to deliver aid to refugees trapped on mountains in northern Iraq.
The jets will carry out surveillance ahead of further airdrops as the UN warned tens of thousands were trapped.
A "small number" of Chinook helicopters would also be sent for "further relief options", the Foreign Office said.
It comes after advances by Islamic State (IS) forces, amid reports of persecution of religious minorities.
Among those reportedly being targeted by the jihadists are Iraqi Christians and members of the Yazidi religious sect.
The UN Refugee Agency said about 35,000 people had escaped from the mountains into Syria and then into the Kurdistan region of Iraq in the past three days.
It said most were "exhausted, dehydrated and many have suffered sun or heat stroke". But it said "an estimated 20,000 to 30,000" Yazidis remained trapped without food, water or shelter.
The UN warned they needed "life-saving assistance", saying an estimated 700,000 Yazidis had been displaced.
Mirza Dinnay, a Yazidi relief worker, who was on board an Iraqi Air Force helicopter evacuating the trapped refugees, described the situation on the ground as "a genocide".
He told the BBC he had seen what looked like hundreds of bodies and said people had fought to get on board the helicopter after they landed.
US forces have carried out a number of air strikes targeting IS militants near Irbil, the capital of Iraqi Kurdistan, while drones have also fired on jihadist fighters.
By James Robbins, BBC diplomatic correspondent
The government is steadily increasing its commitment to the humanitarian mission in northern Iraq, but still resisting pressure to join in US air strikes or put British military boots on the ground.
Some critics have complained the response has been sluggish, others want direct military intervention, and a growing number of MPs are urging the recall of Parliament. So far, the prime minister has been reluctant to encourage the speaker of the Commons to initiate such a debate.
That may be because ministers fear they could not predict the outcome of any eventual vote, after losing last summer's vote over intervention in Syria.
With a new foreign secretary who's been in post less than a month, and two Foreign Office ministers resigning in the past week, the emphasis from Whitehall now is on considering each step with great care.
Tornado jets began leaving RAF Marham in Norfolk just after 14:00 BST (13:00 GMT) - destined for RAF Akrotiri in Cyprus - to help RAF C-130 Hercules cargo planes identify safe areas on the ground to drop supplies.
An RAF aid drop had to be aborted on Sunday because of fears that people on the ground could be injured by the cargo.
A government spokesman later said a "small number" of Chinook helicopters would also be sent to the region for "use if we decide we need further humanitarian relief options".
A No 10 statement said it had also agreed to transport "critical military re-supplies" provided by other countries to Kurdish forces fighting IS militants.
The UK government had continually said operations in Iraq were to provide humanitarian help to refugees.
However, BBC political correspondent Ross Hawkins said the announcement that UK aircraft would transport military supplies to Kurdish forces was of a "rather different order" to simply providing aid.
He said there could now be "more pressure" to recall Parliament to discuss "exactly what it is the UK is doing and exactly what role it sees for itself in this crisis".
The Downing Street statement said three UK aid drops had now taken place in Iraq, adding: "We will continue with these deliveries."
"Meanwhile, urgent planning to get those trapped on the mountainside to safety will continue in the coming days between ourselves and US, the Kurdish authorities and other partners," it added.
It said the UK would also continue to "lead negotiations" on a UN Security Council Resolution that would make clear the "shared determination of the international community" to tackle the threat posed by IS.
BBC defence correspondent Jonathan Beale said it was "inconceivable" that the RAF Tornados would not be armed during flights over Iraq. But he said any prospect of UK planes taking part in strikes "does not seem to be likely at the moment".
He said the UK government would have to discuss any potential plans to carry out airstrikes in Iraq with the government of Cyprus - where the Tornados are based.
It comes amid calls from church figures for the UK to do more to deal with the crisis.
The Rev Rose Hudson-Wilkin, chaplain to Commons Speaker John Bercow, told Channel 4 News the world had to respond to the crisis, adding that "maybe we need to go to the extent of military action".
Cardinal Vincent Nichols, leader of the Roman Catholic Church in England and Wales, told ITV News that "two air drops in four days is maybe not the best we could be doing and I'd like to see that increased".
Meanwhile, former Liberal Democrat leader Sir Menzies Campbell added his voice to calls for Parliament to reconvene to discuss Britain's response.
"The government would be well advised to recall Parliament," he said, to share with MPs "everything that the government has done and - at the moment at least - is intending to do".
But Foreign Secretary Philip Hammond, speaking after a meeting of the government's emergency Cobra committee, said this was not necessary.
By Andrew Plant, BBC correspondent at Cotswold Airport
The pallets of aid here at the UK Aid warehouse are about 1.5 metres cubed in size and they are stacked from floor to ceiling, 10 pallets high. Here and there are empty spaces where aid has already been removed to be dropped in Iraq.
On a table some of the supplies are laid out, including a large white tarpaulin for refugees to shelter from Iraq's blazing sun.
There is what looks like a yellow jerrycan, which can be filled with dirty water. A few pumps of a handle makes fresh water dribble from the other end. The filter will last for around two years, giving thousands of litres of drinking water.
Lastly, there is a solar-powered lamp, which has a multi-head adaptor so it can be plugged into mobile phones to charge them.
That is important for aid agencies, as they need to be able to talk to people on the ground in Iraq to establish where they are and what they need.
On Monday night, RAF crews dropped supplies including 3,180 reusable water purification containers containing a total of 15,900 litres of clean water, and 816 solar lamps that can also be used to charge mobile phones.
The first drop took place on Saturday, delivering 1,200 water containers and 240 solar lanterns.
International Development Secretary Justine Greening said the humanitarian situation in Iraq was "deeply worrying" but UK aid was helping those who needed it.
Ms Greening has also announced that four organisations - Mercy Corps, Save the Children, the International Rescue Committee and Action Against Hunger - are to share the £3m earmarked by the British government for charities operating in Iraq.