The world must take "all possible measures" to prevent a massacre of minorities in Iraq, United Nations human rights experts warned as the UK continued aid drops but resisted calls for military intervention.

With up to 40,000 Yazidis still trapped on a sweltering mountainside by extremist forces of the self-styled Islamic State (IS), the panel warned that a mass atrocity or even genocide could take place "within days or hours".

Around a quarter of a million Iraqis from religious minorities have already fled their homes in the face of "convert or die" ultimatums from the advancing militants, with women executed or taken as slaves and teenagers sexually assaulted, their stark report concluded.

The UN's special rapporteur on minority issues Rita Izsak urged: "All possible measures must be taken urgently to avoid a mass atrocity and potential genocide within days or hours - civilians need to be protected on the ground and escorted out of situations of extreme peril."

Foreign Secretary Philip Hammond is due to chair another meeting of the Government's emergency committee Cobra to discuss the next stage of Britain's response to the crisis.

Last night, RAF aircraft dropped a second round of "essential supplies" to some of those trapped on Mount Sinjar - after a previous attempt had to be abandoned for fear it could hurt those on the ground.

A "small number" of Tornado jets are also being sent to the region so they can be used, if required, to improve the UK's surveillance capability in the region to help the humanitarian effort.

And Downing Street has indicated that the Government is also looking at how it can play a role in getting equipment to Kurdish forces so they are better able to counter IS, formerly known as the Islamic State of Iraq and the Levant (Isis).

But voices continued to be raised in favour of a more direct military intervention - and for MPs to be recalled from their summer break to debate and vote on it.

Tory MP Mark Pritchard became the latest to join politicians and former military chiefs in urging a stronger response, including targeted air strikes and drone attacks but falling short of sending ground forces back to the country.

He predicted a recalled Commons would vote in favour of the escalation.

"The international community can pour as much aid into the region as its likes," he wrote for the PoliticsHome website.

"But unless Islamic State fighters are killed and removed from the battlefield, they will go on killing and committing further atrocities.

"This intervention needs to be in an unequivocal combat role, not just in an intelligence, surveillance and targeting role."

US air strikes on Islamic State positions over recent days have helped Kurdish peshmerga fighters defending Irbil - the capital of the autonomous northern region - to regain some territory from the Islamic State forces, including two towns.

Mr Pritchard said the different circumstances suggested there would be no repeat of MPs' refusal last year to endorse military intervention in Syria.

"I suspect this time Parliament would vote for decisive, precision, and targeted military action in Iraq, with clearly defined rules of engagement."

One of Britain's most senior generals accused the "commitment-phobic" Government of being "terrified" of intervening in the Iraq crisis before next year's general election.

General Sir Richard Shirreff told the Times: "The longer we sit on our hands and prevaricate, the more dangerous the situation is going to become."

But speaking after yesterday's Cobra meeting, Mr Hammond rejected calls for Parliament to be recalled to discuss the crisis and said there were no plans for British military involvement.

"We don't envisage a combat role at the present time," he said.

Prime Minister David Cameron is on a family holiday in Portugal but due back in No 10 later this week.

Downing Street said he was "very much engaged" with the situation despite being abroad and a recall of Parliament was "not on the cards".

The Department for International Development (DfID) said the UK's second aid drop included 3,180 reusable water purification containers filled with 15,900 litres of clean water and 816 solar lamps that can also be used to charge mobile phones.

Christof Heyns, the UN's special rapporteur on extrajudicial, summary or arbitrary executions, said the international community "must do all in their power to support those on the ground with the capacity to protect lives".

His colleague Chaloka Beyani, special rapporteur on the human rights of internally displaced persons, said: "We are witnessing a tragedy of huge proportions unfolding in which thousands of people are at immediate risk of death by violence or by hunger and thirst.

"Humanitarian aid must be delivered quickly and no efforts should be spared to protect all groups forcefully displaced by this conflict."

Rashida Manjoo, special rapporteur on violence against women, warned: "We have reports of women being executed and unverified reports that strongly suggest that hundreds of women and children have been kidnapped, many of the teenagers have been sexually assaulted, and women have been assigned or sold to IS fighters as 'malak yamiin' or slaves.

"Such violations are crimes against humanity that must be stopped and punished."

The developments came amid increased political turbulence in the Iraqi capital where the country's president did not choose incumbent prime minister Nouri Maliki as the man to form a new government.

Deputy speaker of parliament Haider Ibadi has been given 30 days to present a new government for MPs' approval, with the West calling for an inclusive administration to help combat IS.

US president Barack Obama called Mr Ibadi's nomination in place of Mr Maliki - who is accused of fuelling separatism during his years in power - a "promising step forward".