The United States said Tuesday it would not coordinate with the government of President Bashar Assad on targeting Islamic State militants on Syrian territory, as preparations for possible air strikes gathered pace.

President Barack Obama meanwhile warned that it would not be a quick or easy job to root out the IS "cancer," but said extremists responsible for the murder of U.S. journalist James Foley would be hunted down and America would not forget their crime.

The United States said Monday that it had asked Qatar in advance not to pay a ransom for the release of U.S. hostage Peter Theo Curtis, who was freed by an Islamic rebel group in Syria.

Syrian war planes carried out a series of air raids Tuesday on positions held by the Islamic State jihadist group in eastern Deir Ezzor province, an NGO and state media said.

"Syrian army forces targeted headquarters and warehouses storing weapons and ammunition belonging to the terrorist group known as the Islamic State... destroying them completely," Syria's state news agency SANA reported.

Interior Minister Nouhad al-Mashnouq played down on Tuesday the confessions of al-Nusra Front leader, whose arrest sparked clashes between Islamist gunmen with the Lebanese army earlier in August.

“I don't think that (Imad) Jomaa's confessions would thwart the ongoing endeavors to release the kidnapped soldiers,” Mashnouq said in comments published in al-Liwaa newspaper.

Signs the United States may strike Islamic State militants in their Syrian stronghold reveal a shift in the politics of foreign war in Washington, after the trauma of the post-Iraq era.

The United States is poised to send spy planes into Syria to track Islamic State jihadists whose advances have sparked international concern and American air strikes in neighboring Iraq.

A U.S. official confirmed the plans after Syria said on Monday it was willing to work with the international community, including Washington, to tackle extremist fighters.

The White House said Monday that President Barack Obama had so far made no decision on whether to launch air strikes on Islamic State militants in Syria.

"The president has not made any decision to conduct military action in Syria," White House spokesman Josh Earnest said, amid rising expectations of U.S. action following American attacks against the jihadist group in Iraq.

Murdered U.S. hostage James Foley told his parents of his life imprisoned with 17 other captives in a Syrian dungeon, in a moving smuggled letter.

The 40-year-old freelance reporter -- whose death was revealed last week in a video released by militants from the so-called "Islamic State" -- taught a fellow hostage to memorize his message.

The U.S. military's top general believes Islamic State extremists will "soon" pose a threat to America and Europe and that an international coalition will be needed to confront it, his spokesman said Monday.

U.S. commanders are preparing possible "options" to counter IS jihadists both in Iraq as well as Syria, according to General Martin Dempsey's spokesman, Colonel Ed Thomas.

Qatar, which mediated the release of a U.S. hostage held by al-Qaida in Syria, seeks to prove its role in confronting the Islamist radicals it has been accused of supporting, experts say.

The Qatari foreign ministry, in a statement, said the tiny gas-rich nation had "succeeded in releasing American journalist Peter Theo Curtis," after he spent 22 months in captivity.