BAGHDAD  - President Barack Obama urged a broad front against the Islamic State as Nato leaders gathered in Wales Thursday with Britain and France weighing joining US air strikes in Iraq. Pressure on Western governments to take firm action against the militants in Iraq and Syria has risen sharply after the brutal videotaped execution of a second American journalist and a threat to kill a captive British aid worker. The beheading of 31-year-old reporter Steven Sotloff came amid mounting evidence from the United Nations and human rights groups of the scale of the atrocities committed by IS fighters in northern Iraq and eastern Syria against ethnic and religious minorities.

“We know that if we are joined by the international community, we can continue to shrink (IS’s) sphere of influence, its effectiveness, its financing, its military capabilities,” Obama said.

“And the question is going to be making sure we’ve got the right strategy, but also making sure that we’ve got the international will to do it,” he said in Estonia before heading to the summit in Wales.

NATO chief Anders Fogh Rasmussen said on Thursday that NATO would “seriously” consider any request from Iraq for assistance in a war against insurgents from the radical Islamic State. “We will discuss what individual allies and what NATO can do to counter the threat from the terrorist organisation, so-called Islamic State,” he said before the start of a two-day summit of leaders from the Western military alliance in Newport, Wales.

“We haven’t received any request for NATO engagement. I’m sure that if the Iraqi government were to forward a request for NATO assistance, that would be considered seriously by NATO allies,” the secretary general said. With one of its nationals also under threat of beheading, summit hosts Britain said it would not rule out taking part in air strikes if necessary.

“I can assure you that we will look at every possible option to protect this person,” Foreign Secretary Philip Hammond said. And French President Francois Hollande likewise raised the prospect of a military response to the threat posed by IS. “The head of state underlined the importance of a political, humanitarian and if necessary military response in accordance with international law,” his office said. Obama pledged that justice would be done to the killers of 31-year-old reporter Steven Sotloff, wherever they hid and however long it took.

Obama will lead a UN Security Council session on the threat of foreign fighters in Iraq and Syria on September 25, a US official said Wednesday.

His Secretary of State John Kerry revealed he was working to forge a global coalition to fight the “mediaeval savagery” of Islamic militants terrorising a swathe of Syria and Iraq. IS posted video footage on the Internet of Sotloff’s beheading, confirmed as authentic by Washington, which sparked outrage around the world.

It said the journalist’s killing, which came on the heels of its beheading last month of another US reporter, James Foley, was in retaliation for expanded US air strikes against its fighters in Iraq during the past week.

It warned that a British hostage would be next unless London backs off from its support for Washington’s air campaign. Britain has so far only carried out reconnaissance flights in support of the US air campaign from its base in Cyprus. France has joined Britain and the United States in air-dropping relief supplies to stricken civilians in northern Iraq, while other Nato allies, including Germany, have joined in flying arms supplies to Kurdish forces battling IS. Obama said Washington was determined to halt the IS threat but warned it would depend on close cooperation with partners in the region.

Meanwhile, Russia’s foreign minister warned Nato not to offer Ukraine membership of the alliance as it gathered for a summit on Thursday and told the United States not to try to impose its will on the former Soviet republic.

Sergei Lavrov also urged Kiev and pro-Russian rebels fighting Ukrainian forces in eastern Ukraine to back peace moves outlined by President Vladimir Putin and avert what he said could be a large-scale crisis in the heart of Europe.

Putin unveiled the seven-point plan on Wednesday, the eve of a Nato summit at which the crisis in Ukraine will be discussed.

“It is precisely at such a moment when a chance has emerged to start solving specific problems between Kiev and the militias that some sections of the Kiev authorities make demands for Ukraine to drop its non-aligned status and start joining NATO,” Lavrov said at talks with the Organization for Security and Cooperation in Europe, a rights and security group.

“It’s a blatant attempt to derail all efforts aimed at initiating a dialogue on ensuring national reconciliation.”