The United States has also called the Paris attacks an 'act of war,' but Washington's options for battling Islamic State are limited. A major change in strategy is unlikely for the time being, experts say.
It's one of the few concrete changes in US strategy in the wake of the terrorist attacks in Paris. Washington has launched a joint operation with NATO ally Turkey to secure the Syrian border. The effort aims to stop the flow of weapons and Islamic State fighters going into and out of the war-torn country.
"The entire border of northern Syria, 75 percent of it has now been shut off, and we are entering an operation with the Turks to shut off the other remaining 98 kilometers [61 miles]," US Secretary of State John Kerry told CNN on Tuesday.
Prior to the attacks in Paris, a small portion of the border had been left open so arms could flow to rebels fighting against President Bashar al-Assad's government, according to Joshua Landis, an expert on Syria and US foreign policy at the University of Oklahoma. Devastating attacks in Paris - as well as Beirut and Ankara - have changed the Turkish and American calculus, Landis said.
"What America is deciding do is to stop arming rebels and keep the Saudis and Qataris from arming them too and starve all those rebels of arms," Landis, who is also the editor of Syria Comment, told DW.
"That's what's ultimately going to stop ISIS and al Qaeda," he said, using an alternative acronym for "Islamic State." "They can only thrive if there's open borders."
'Intensification' of current strategy
Otherwise, US President Barack Obama is largely sticking with his current strategy - launching airstrikes and limited use of Special Forces while cooperating with local forces, such as the Kurds.
"There will be an intensification of the strategy we put forward, but the strategy that we are putting forward is the strategy that ultimately is going to work," Obama said after meeting with world leaders at the G20 summit in Turkey on Monday. "But as I said from the start, it is going to take time."
White House spokesman Ben Rhodes told ABC News on Sunday that the United States, like France, considers the attacks in Paris an "act of war." But the US president ruled out launching a large ground operation in response, citing the limitations of American power in Afghanistan and Iraq over the past 14 years.
Whose boots on the ground?
According to Landis, the president's current strategy of leveraging local forces to wage the ground war against Islamic State has reached a dead end. Washington doesn't have reliable allies on the ground other than the Kurds, who shouldn't be encouraged to move beyond their northern homelands, he said.
"If America tries to arm up the Kurds to take Arab majority territory away from ISIS, you're just going to get an ethnic war where Kurds are dominating Arabs," Landis said. "And you'll impose one injustice in place of another."
But airstrikes alone won't destroy the Islamic State, said Majid Rafizadeh, president of the International American Council. The UN General Assembly should vote to deploy an international military coalition, rooted in the principle of Responsibility to Protect (R2P), Rafizadeh said. R2P calls on the international community to protect civilians from atrocities - with military force if necessary.
"There is a need for a comprehensive strategy which focuses on tactical, intelligence, strategic cooperation among several states as well as joint boots on the ground to effectively defeat the Islamic State," Rafizadeh told DW via email.
Deploying large numbers of American and Western troops in Syria would be a mistake, said Brian Katulis, an expert on the Middle East at the Center for American Progress. Washington should instead leverage its military aid to the Gulf nations and encourage them to commit Arab troops to destroying Islamic State, he said.
'Partitioning the country'
On the diplomatic front, the United States and Russia did manage to come find some common ground during negotiations in Vienna over the weekend.
They agreed to an immediate ceasefire followed by negotiations between the Assad regime and the fractious rebels beginning in January. An interim government would write a new constitution during a sixth-month transition period and Syrian elections would be held in 18 months.
Moscow and Washington disagree on the fate of Syrian President Assad and what groups should be labeled terrorists - and excluded from the ceasefire - other than Islamic State and al Qaeda. On Tuesday, Secretary of State Kerry said the implementation of the ceasefire was only weeks away.
"Ceasefire is another word for partitioning the country, which still leaves America with a terrible problem because Russia has all the good bits of Syria - the West coast and the cities," Landis said.
"America is going to be caught holding all of the ISIS controlled territory," he added. "And then what do you do with ISIS?"