Islamic State in Iraq and Syria has released a new video warning the United States that ISIS fighters are ready and waiting if President Barack Obama sends troops, as his top general said may happen if the current strategy of airstrikes fails.

The slick, 52-second video entitled Flames of War shows militants blowing up tanks and images of wounded U.S. soldiers. It then shows a clip of Obama saying that combat troops will not be returning to Iraq, ending with a text overlay that reads "fighting has just begun."

The video's timing, released late Tuesday, suggested it was a response to Gen. Martin Dempsey, the chairman of the Joint Chiefs of Staff, who said in testimony to the Senate Armed Services Committee that if the current Iraq strategy doesn't prevail, he may recommend the use of ground troops.

"To be clear, if we reach the point where I believe our advisers should accompany Iraqi troops on attacks against specific ISIL targets, I will recommend that to the president," Dempsey declared. (He referred to the militants by an alternative name.)

Obama said Wednesday afternoon that U.S. forces in Iraq "do not and will not" have a combat mission as part of the effort against ISIS militants. Speaking in Tampa, Fla., after a briefing from officers at U.S. Central Command, he said the fight against ISIS requires a broad coalition of countries.

Some countries will assist the U.S. with airstrikes and others will help train forces, he said.

The video was released by the Islamic State group's Al Hayat media centre and was portrayed as a trailer for a longer video to follow. The group operates a sophisticated propaganda machine with slick production techniques, using social media to broadcast a stream of battles, bombings and beheadings to a global audience.

The strategy is aimed at terrorizing opponents at home and winning recruits abroad. The group recently released three videos showing the beheading of two American journalists and a British aid worker.

Reuters reported Wednesday that ISIS has gone underground in its Syrian stronghold since Obama authorized U.S. airstrikes on the group in Syria, disappearing from the streets, redeploying weapons and fighters, and cutting down its media exposure.

In the city of Raqqa, 450 kilometres northeast of Damascus, residents say Islamic State has been moving equipment every day since Obama signalled on Sept. 11 that air attacks on its forces could be expanded from Iraq to Syria.

SYRIA-CRISIS/WARPLANE

An Islamic State of Iraq and Syria (ISIS) militant carries a piece of wreckage from a Syrian war plane after it crashed in Raqqa, in northeast Syria on Turesday. (Stringer/Reuters)

As the United States tries to assemble a coalition to fight ISIS, the jihadist group appears to be trying to leave as much uncertainty as possible about its strategy.

Facing U.S. air strikes in Iraq, ISIS fighters abandoned heavy weaponry that made them easy targets and tried to blend into civilian areas. In anticipation of similar raids in Syria, the group may already be doing the same.

In Raqqa, the group has evacuated buildings it was using as offices, redeployed its heavy weaponry, and moved fighters' families out of the city.

"They are trying to keep on the move," said one Raqqa resident, communicating via the Internet and speaking on condition of anonymity because of safety fears. "They have sleeper cells everywhere," he added.

"They only meet in very limited gatherings."

The top U.S. general promised on Tuesday "a persistent and sustainable campaign" against ISIS in Syria, and Washington is probably already watching its positions in Raqqa. Obama approved surveillance flights over Syria last month, and footage taken by activists earlier this month appeared to show an American-made drone over the city.

The militants are not dormant; the Syrian Observatory for Human Rights, which tracks violence in the country's civil war, said they had shot down a Damascus government war plane near Raqqa using anti-aircraft guns.

Main base

However, another resident said: "Islamic State is now carrying out tactical defensive moves by relocating their assets to different places so that their heavy weaponry is not all concentrated in one place."

Raqqa and the surrounding province is Islamic State's main base in Syria. Last month, its fighters drove the final government forces from the area when it seized an air base.

Since seizing the Iraqi city of Mosul in June, the group has also extended its control over neighbouring Deir al-Zor province, which borders Iraq. Making good on its promise to redraw the Middle East, ISIS has declared a new province including territory on both sides of the frontier.

In Raqqa, Islamic State had taken charge of many aspects of civilian life, managing everything from traffic to bakeries in an effort to establish a state run according to its own, radical interpretation of Islam.

ISIS has been trying to give a sense of business as normal even as it has reduced its presence in the streets, said another resident of the city whose population numbered about 200,000 before the civil war. "They are giving the impression they don't care," the resident said.

"These days the fighters are not deployed heavily on the streets. Only those who have to are appearing. The streets are empty and the people are worried and scared."

Fear drives up dollar

Some activists did appear on the outskirts of Raqqa on Tuesday. They were pictured collecting wreckage of the downed Syrian war plane and loading it into the back of a truck flying the group's black flag.

Since Obama's speech, shops in Raqqa have been closing early and the value of the U.S. dollar has jumped in the local hard currency market, residents said. Dozens of people have left the city, though there has been no sign of mass migration.

While preparing for an attack, ISIS has also been trying to promote its cause among residents. Some already express support for the group whose rule has brought a modicum of stability, albeit in a hardline form.

A 14-point statement distributed in recent days reminded residents of ISIS rules such as its ban on smoking and drinking, and requirement for women to cover up and stay at home. It also warned that anyone who dealt with President Bashar al-Assad's government would face death.

But the statement also tried to promote the group, telling residents they would see "the great difference" between ISIS rule and that of the "oppressive secular government" – a reference to Assad. "Live joyfully and in plenty in an Islamic government," declared the statement, which was obtained by the Syrian Observatory for Human Rights.

In one of the few responses to Obama's announcement, an ISIS supporter warned of attacks on the United States and its allies if they continued to carry out military action against the group, the SITE monitoring service said on Tuesday.

With files from Reuters