The Foreign Secretary told MPs he had held a phone conversation with Iran’s foreign minister Mohammad Javad Zarif, amid reports Tehran is considering giving military support to the Shia-led administration in neighbouring Iraq, which is under assault from militants from the Sunni-dominated Islamic State of Iraq and the Levant, known as Isis.
Mr Hague confirmed the UK would not be involved in any military action, a statement that was welcomed by Labour shadow foreign secretary Douglas Alexander.
As UK politicians joined international condemnation of apparent massacres of Iraqi army troops by the extremists, US secretary of state John Kerry said American drone strikes were an option under consideration to halt the insurgency.
He also said the Obama administration was willing to hold talks with Iran on military co-operation between the two countries over Iraq.
Mr Hague said it was “inevitable” British citizens were involved with the Isis rebels in Syria and Iraq. He said: “We estimate the number of UK-linked individuals fighting in Syria to include approximately 400 British nationals and other UK-linked individuals who could present a particular risk should they return to the UK.”
He said “some of these, inevitably” were fighting with Isis.
Mr Hague said: “We are taking action in three areas: promoting political unity among those who support a democratic Iraqi state and stability in the region; offering assistance where appropriate and possible, and alleviating humanitarian suffering.
“We have made it clear this does not involve planning a military intervention by the United Kingdom.”
Giving further details of what UK involvement could be, he said: “We are discussing with the Iraqi government areas for co-operation, including the possibility of offering counter-terrorism expertise.
“We are also providing consular assistance to a small number of British nationals who have been affected. For this purpose a UK MoD operational liaison and reconnaissance team arrived in Baghdad on Saturday to help assess the situation on the ground and assist the embassy on contingency planning.”
The US has sent an aircraft carrier to the Gulf in advance of potential air strikes.
Mr Hague told MPs: “The United States, which is the country with the most appropriate assets and capabilities, is considering a range of options that could help the Iraqi security forces push back on Isil [Isis] advances.
“President Obama has been clear that action taken by the United States will only succeed if accompanied by a political response from the Iraqi government.”
At the weekend, former prime minister Tony Blair called for a tough response to the extremist insurgency – arguing that it had been caused by a failure to deal with the Syria crisis, not the invasion by US and British forces 11 years ago.
For Labour, Mr Alexander said yesterday it would be “facile” to blame the current crisis purely on the Iraq war, but said the fears of those who opposed the military intervention had been vindicated.