The Islamic State released a video Monday threatening to attack the United States saying, "We will drown all of you in blood". Meanwhile, as US hits additional Islamic State targets in northern Iraq, Syria's President Bashar Assad has increased attacks on the group's forces in Syria.
The Islamic State militant group that has seized large parts of Iraq and drawn the first American air strikes since the end of the occupation in 2011 has warned the United States it will attack Americans "in any place" if the raids hit its militants.
The video, which shows a photograph of an American who was beheaded during the US occupation of Iraq and victims of snipers, featured a statement which said in English "we will drown all of you in blood".
US airstrikes in northern Iraq have helped Kurdish fighters take back some territory captured by Islamic State militants, who have threatened to march on Baghdad.
The latest advance by the Islamic State, an al Qaeda offshoot, sent tens of thousands of members of the Yazidi ethnic minority and Christians fleeing for their lives and alarmed the Baghdad government and its Western allies.
Yazidi refugees escaping ISIS (Photo: Reuters)
Unlike al-Qaeda, Islamic State has so far focused on seizing land in Iraq and Syria for its self-proclaimed caliphate, not spectacular attacks on Western targets.
As the US military strikes, Syrian President Bashar Assad's forces have significantly stepped up their own campaign against militant strongholds in Syria, carrying out dozens of airstrikes against the group's headquarters in the past two days.
ISIS militants in Syria (Photo: AP)
While the government in Damascus has long turned a blind eye to the Islamic State's expansion in Syria - in some cases even facilitating its offensive against mainstream rebels - the group's rapid march on towns and villages in northern and eastern Syria is now threatening to overturn recent gains by government forces.
While Islamic State militants have so far concentrated their attacks against the Western-backed fighters seeking to topple Assad, they have in the past month carried out a major onslaught against Syrian army facilities in northeastern Syria, capturing and slaughtering hundreds of Syrian soldiers and pro-government militiamen in the process.
On Monday, Islamic State fighters were closing in on the last government-held army base in the northeastern Raqqa province, the Tabqa air base, prompting at least 16 Syrian government airstrikes in the area in an attempt to halt their advance.
In the northern city of Aleppo, there is a sense of impending defeat among mainstream rebels as Islamic militants systematically routed them last week in towns and villages only a few kilometers (miles) north of the city. An Islamic State takeover of rebel-held parts of Aleppo also would be disastrous for Syrian government troops who have been gaining ground in the city in past months.
"I think they (Syrian government) are finally realizing that their Machiavellian strategy of working with the Islamic State group against the moderates did not work so well, and so they have started to fight it," said Andrew Tabler, a senior fellow at the Washington Institute for Near East Policy.
But in hitting hard against the Islamic State group, Assad has another motive. His aerial bombardment of militant strongholds in Syria in a way mirrors that of the US military's airstrikes against extremists across the border in Iraq.
Analysts say Assad's strikes aim at sending a message that he is on the same side as the Americans, reinforcing the Syrian government's longstanding claim that it is a partner in the fight against terrorism and a counterbalance to extremists. That comes after the US itself nearly bombed Syria after it blamed Assad's forces for a chemical weapons attack on rebel-held areas near Damascus last August.
"Assad would surely love to regain international acceptance via a 'war on terror' and maybe that is his long-term plan, in so far as he has one," Syria analyst Aron Lund said.
Even while going against the Islamic State in Iraq, US officials have shown little appetite for striking at the same militants in Syria. Assad knows that the US administration doesn't have much of a plan for Syria, except to muddle through the mess created by more than three years of civil war.
Most of all, however, Assad can simply no longer afford to ignore the growing threat of the Islamic State now that it has started attacking his own forces.
Since July, following their blitz in Iraq and after they declared a self-styled caliphate straddling the Iraq-Syria border, Islamic State fighters have methodically gone after isolated government bases in northern and eastern Syria, killing and decapitating army commanders and pro-government militiamen.
The attacks started with a devastating onslaught on the al-Shaer gas field in Homs province in which more than 270 Syrian soldiers, security guards and workers were killed. Last month, the jihadis overran the sprawling Division 17 military base in Raqqa province, killing at least 85 soldiers. Two weeks later, Islamic State fighters seized the nearby Brigade 93 base after days of heavy fighting.
They now are closing in on Tabqa air base. Activists on Monday reported intense clashes between government troops and Islamic State fighters on the edge of the villages of Ajil and Khazna near Tabqa. The Raqqa Media Center, an activist collective, said the Islamic State captured four villages near the air base, including Ajil.
"They will stop at nothing. If things continue the same way it's only a matter of time before the Islamic State seizes Aleppo," said Abu Thabet, an Aleppo rebel commander. He said the jihadis were now looking to take the rebel stronghold of Marea, to be followed by the Bab al-Salama border crossing with Turkey, which would be a major prize and source of money.
Oubai Shahbandar, a Washington-based senior strategist for the Western-backed opposition Syrian National Coalition group, called Assad's airstrikes against the Islamic States superficial, saying the Western-backed rebels were the only force truly confronting the jihadis.
He shrugged off any suggestion that Assad and the West share a common enemy in the Islamic State group.
"The choice for the West is clear," he said. "Assad turned Syria into a springboard for terror, while the opposition leads the anti-Islamic State resistance."
Reuters and Associated Press contributed to this report.