The Syrian government said it was "reprehensible" that Washington was prepared to enter a dialogue with the Islamic Front, which comprises six major Islamist rebel groups and which it considers a terrorist organization.

The developments highlighted the difficulty for Washington in engaging with the faction-ridden rebels in Syria's civil war.

With five weeks to go until U.N. peace talks in Geneva, it is unclear who will represent the opposition, and the clock is ticking towards a Dec. 27 deadline set by U.N. mediator Lakhdar Brahimi to name delegations.

"The Islamic Front has refused to sit with us without giving any reason," U.S. Syria envoy Robert Ford told Al Arabiya television, a day after Secretary of State John Kerry said talks might take place.

"We are ready to sit with them because we talk to all parties and political groups in Syria," said Ford, speaking in Arabic.

Hostility towards the United States among al-Qaida-linked groups operating in Syria could make it hard for the newly formed Islamic Front to engage publicly with Washington.

Any open cooperation could put it on a collision course with the powerful and more radical Islamic State in Iraq and the Levant (ISIL), al-Qaida's leading franchise in the region.

In a statement released by the Syrian state news agency, a foreign ministry spokesman said dialogue with the Islamic Front contradicted U.S. and international commitments to combat terrorism as well as "international pledges that terrorist organizations would not be given the chance to participate in the Geneva conference."

The long-delayed talks in Switzerland are meant to discuss a political transition to lead the country out of a civil war that has killed well over 100,000 people.

The Islamic Front has overshadowed the more moderate Free Syrian Army brigades, which are led by the Supreme Military Council (SMC) and backed by Western and Arab powers.

The Front has rejected the authority of the SMC, the main opposition's military wing, and last week seized control of SMC weapons depots in northern Syria.

The Islamic Front "agrees in principle, strategy, and goals with the Nusra Front," the Syrian foreign ministry statement said, referring to an al-Qaida-linked rebel group that the State Department last year designated as a terrorist organization.