US President Barack Obama delivers a speech about US - Estonia relations, as well as the situation in Ukraine, at Nordea Concert Hall in Tallinn, Estonia, September 3, 2014. US President Barack Obama underscored Washington's commitment to the security of NATO allies, announcing additional US planes to police the skies over Europe's eastern flank bordering Russia. AFP PHOTO / SAUL LOEB (Photo credit should read SAUL LOEB/AFP/Getty Images)

WASHINGTON (AP) — World leaders, nervously eyeing a growing threat from Islamic State militants, will seek to build a united front this week against the violent extremist group and keep it from creeping beyond its borders.

Yet international plans to counter the Islamic State group — with combined military might, diplomatic pressure on abetting partners and economic penalties — pale in comparison to the extremists' ruthlessness and command of the swath of land it controls across parts of northern Syria and Iraq. For the second time in as many weeks, Islamic State militants released a video showing the beheading of an American journalist, and governments from Britain to Saudi Arabia to Australia warned of the potential of their citizens joining the fight — and then bringing the violence back home.

"Our objective is clear, and that is to degrade and destroy ISIL so that it's no longer a threat," President Barack Obama said Wednesday during a visit to Estonia, using an alternate acronym for the Islamic State. He later headed to Wales for an annual meeting of leaders of the NATO military alliance.

Separately, during an appearance in Maine, Vice President Joe Biden declared that the U.S. will pursue the militants to "the gates of hell."

In Wales, British Prime Minister David Cameron said he was considering joining a nearly monthlong U.S. airstrike campaign in Iraq against the Islamic State group, adding to military aid that London has already approved.

"We'll always ask ourselves what is in our national interest," Cameron said, according to Britain's Guardian newspaper. "Not ruling things out, but going forward in a deliberate, sensible, resolute way."

Obama and Cameron co-authored an op-ed published Wednesday evening in the Times of London, accusing the militant group of “brutal and poisonous extremism."

“If terrorists think we will weaken in the face of their threats they could not be more wrong. Countries like Britain and America will not be cowed by barbaric killers,” they wrote. “We will be more forthright in the defense of our values, not least because a world of greater freedom is a fundamental part of how we keep our own people safe.”

And in the Mideast, the United Arab Emirates called for a coordinated international effort to tackle the "global scourge" of terrorism, raising particular concern about the threat posed by Islamic State fighters.

The heightened urgency reflected fears that the Islamic State was growing stronger in its quest to create a caliphate territory in the Mideast and systematically kill any who resist. The group is considered even more merciless toward its enemies than the al-Qaida terror network, and intelligence officials across the world warn that it may soon seek to seed its violence beyond its declared borders.

So far, the Islamic State has beheaded two American journalists it held captive for what the militants called payback for more than 120 U.S. airstrikes on its assets in northern Iraq since Aug. 8. Journalists James Foley and Steven Sotloff were two of what the State Department has described as "a few" Americans still being held hostage by the group. The Islamic State also had threatened to kill a British man it is holding hostage.

In a statement Wednesday, a family spokesman said Sotloff dedicated his life to portraying the suffering of people in war zones, but was "no hero."

Family spokesman Barak Barfi told reporters gathered outside the family's suburban Miami home that Sotloff "tried to find good concealed in a world of darkness," and to give voice to the weak and suffering in the Arab world. Barfi said Sotloff was "no war junkie," but was drawn to the stories of the turbulent Middle East, and his family has pledged to "not allow our enemies to hold us hostage with the sole weapon they possess — fear."

Sotloff, a 31-year-old who freelanced for Time and Foreign Policy magazines before he was captured in Syria a year ago, was also an Israeli citizen. But his Jewish faith and Israeli citizenship were not widely known before his death — in part because Israel's military censor apparently kept a lid on the story for his safety — and his killers are not believed to have known about his background.

In Washington, Obama administration officials maintained that the U.S. will not launch a ground war against the Islamic State militants. But they stopped short of ruling out airstrikes against the group in its safe haven in Syria, as the U.S. has resisted for years.

Obama has "been clear that we're not going to be limited by geography," State Department spokeswoman Jen Psaki said. She described a range of actions being considered against the Islamic State, and noted that decisions and discussions were ongoing.

Over the past day, Secretary of State John Kerry and other administration officials have reached out to leaders from Australia, the UAE, Jordan, Qatar, Saudi Arabia, Italy and Israel to discuss how to combat the Islamic State. Psaki said the discussions focused on what each country might contribute — including weapons, humanitarian aid and other resources — and noted that some nations already have.

Obama "wants to build an international coalition," Psaki said. "That's not going to be overnight. We need capabilities from many countries."

The push also mirrors concerns that the Islamic State will lure foreigners to the fight — who will then return home to launch attacks.

A compilation of government estimates shows more than 2,000 people with European passports have fought or are fighting in Syria and Iraq, with most looking to join the Islamic State group.

In Australia, officials said Prime Minister Tony Abbott has mobilized public support for fighting the militants, and earlier this week singled out citizens and their supporters as a growing threat to his nation's security.

"People who kill without compunction in other countries are hardly likely to be law-abiding citizens should they return to Australia," Abbott told Parliament on Monday.

U.S. intelligence officials also believe a number of Americans seek to join the battle.

The Islamic State and other extremists in Syria "threaten our people and our interests" in the Mideast, Matthew Olsen, the retiring director of the National Counterterrorism Center, told a think-tank audience in Washington on Wednesday. "Left unchecked, they will seek to carry out attacks closer to home."

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Associated Press writers Eric Tucker in Washington, Julie Pace in Tallinn, Estonia, Tia Goldenberg and Aron Heller in Jerusalem, David Fischer in Miami, Adam Schreck in Dubai, United Arab Emirates, Lori Hinnant in Paris and Raphael Satter in London contributed to this report.

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Follow Lara Jakes on Twitter at: https://twitter.com/larajakesAP