In Jordan, the army dispatched reinforcements to its border with Iraq last week to boost security, while in Lebanon heavily armed police busted a purported sleeper cell allegedly linked to the group, known as the Islamic State of Iraq and the Levant, in raids on two hotels in central Beirut.
The region has warily watched the Islamic State's expansion over the past year across much of northern and eastern Syria. But the group's audacious offensive this month in neighboring Iraq, aided by Sunni tribal fighters and former members of Saddam Hussein's Baath party, threatens to redraw the Middle East map - putting a host of governments on alert.
U.S. Secretary of State John Kerry, who is traveling through the Middle East, warned during a stop in Cairo that the Islamic State has become "a threat not only to Iraq, but to the entire region."
Topping the list of concerned nations are Jordan and Lebanon, two countries already grappling with fallout from the Syrian civil war. The urgency of the matter was laid bare after Islamic State fighters captured the Iraqi side of the border with Jordan on Sunday.
The Islamic State has never explicitly stated its desire to expand into either country. But it openly aims to create an Islamic state that encompasses Iraq and Greater Syria, also known as the Levant - traditional names that refer to a swath of land that includes Syria, Jordan, Lebanon, Israel and the Palestinian territories.
An Islamic State propaganda video released last week featuring five fighters from Britain and Australia underlined how far the group is willing to go.
"Look at the soldiers - we understand no borders," says one of the men in the 13-minute clip posted online. "We have participated in battles in Sham (Syria), and we will go to Iraq in a few days, and we will fight there and come back, and we will even go to Jordan and Lebanon, with no problems - wherever our sheik wants to send us."
The Islamic State's estimated 10,000 fighters already have their hands full in Iraq and Syria, and there's no indication that the group has any immediate designs on Jordan or Lebanon. But governments in both countries are eager to assure their anxious publics.
In Amman, officials are clearly concerned. Jordan's interior minister, Hussein al-Majali, told lawmakers last week that the kingdom is "surrounded by extremism," and that the army has fully deployed along the country's 110-mile (180-kilometer) frontier with Iraq. An AP journalist saw additional reinforcements, including tanks and armored personnel carriers, traveling toward the border on Sunday.
Jordan, a close U.S. ally with a well-equipped and well-trained military, would present a far more formidable foe than Iraq's demoralized army, making any cross-border foray unlikely, analysts say.
"What is most worrisome is that radical groups may already have cells inside Jordan among their supporters," said Ramzy Mardini, a nonresident fellow at the Atlantic Council, based in Amman. "Militants in Syria have conducted attacks in several capital cities in the region of neighboring states. There's much concern that Amman isn't immune from experiencing the same."
Extremists have targeted Jordan before. The Islamic State's precursor, known as al-Qaida in Iraq, was founded by a Jordanian, Abu Musab al-Zarqawi. Under his leadership, the group carried out a triple bombing on Amman hotels in 2005 that killed more than 50 people.
Jordan is home to a growing movement of jihadists and ultraconservative Salafis, Mardini said. Hundreds of Jordanians are known to have traveled to Syria to fight in the uprising against President Bashar Assad. Some have joined extremist groups, including the Islamic State.
But a leader of Jordan's Salafis, Mohammed Shalabi, played down the Islamic State's presence in the country.
"There are many people in Jordan who like the Islamic State, but on the ground, they don't have organized people or members in Jordan," said Shalabi, who is also known as Abu Sayyaf.
Still, there is evidence that the group has established at least a base presence. In Maan, in southern Jordan, some 200 supporters of the Islamic State held protests after Friday prayers, carrying banners that declared the city the "Fallujah of Jordan," a reference to the Iraqi city that has been a militant hotbed.
Maan suffers from high unemployment, and during protests over the past year residents have called for the downfall of Jordan's King Abdullah II and clamored for jobs. It is, to a degree, that sort of discontent that the Islamic State has played off of in Iraq to garner support, although there the grievances have a heavy sectarian hue, with Iraq's Sunnis feeling marginalized by the Shiite-dominated government.
Despite economic malaise and pockets of discontent, Jordan would be a stretch for the Islamic State to make significant inroads.
"The Jordanian population is not supportive of these people, the overwhelming majority of the population," said Marwan Muasher, vice president of the Carnegie Endowment and a former Jordanian foreign minister. "We have a very strong army and intelligence service, so you cannot equate the threat that they posed to a state like Syria or even Iraq to the one that they might pose to Jordan."
In the end, he said, the Islamic State is "a security nuisance, but it's not an existential threat" to Jordan.
In Lebanon, it might be something in between.
As in Iraq, there is a large segment of Lebanon's Sunni community that is angry over the treatment of their brethren in Syria, where the rebellion against Assad is dominated by Sunnis. The Lebanese Shiite Hezbollah group's armed intervention in Syria to support Assad has only further stoked those sectarian hatreds.
A series of bombings struck predominantly Shiite districts in Beirut's southern suburbs over the past year. The Islamic State did not claim responsibility for the attacks - groups tied to al-Qaida did - but the bombings showed Lebanon to be a fertile environment for Sunni extremism.
On Friday, security forces dressed in gray camouflage raided two hotels in the bustling Hamra district of Beirut, arresting 17 suspected Islamic State members. A Lebanese security official, who spoke on condition of anonymity in line with regulations, confirmed the arrests netted suspected members of the group.
On the same day, a suicide bomber detonated his vehicle near a police checkpoint in eastern Lebanon, killing one person and wounding 20. It was not clear if the two incidents were related. But the bombing - the first since March - along with the security dragnet in and around Beirut sparked fears of renewed violence in the country.
All but one of the 17 suspects rounded up in Beirut have since been released, according to security officials. While the hotel raids may have largely been for show, they still underscore how fears of the Islamic State resonate in Lebanon.
Akour reported from Amman, Jordan.