Islamist rebels fighting President Bashar al-Assad's forces in Syria have rejected overtures from the United States to sit down and talk, a senior US diplomat said.
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 Damascus considers a terrorist organisation.
The developments highlighted Washington's difficulties in engaging with the faction-ridden rebels in Syria's civil war.
With five weeks to go before UN peace talks convene in Geneva, it is unclear who will represent the opposition, and the clock is ticking towards a December 27 deadline set by UN mediator Lakhdar Brahimi for both sides to name their delegations.
"The Islamic Front has refused to sit with us, without giving any reason," US Syria envoy Robert Ford told Al Arabiya television, speaking in Arabic, a day after Secretary of State John Kerry said such talks might take place.
"We are ready to sit with them because we talk to all parties and political groups in Syria," Mr Ford said.
The Islamic Front has overshadowed the more moderate Free Syrian Army, which is formally led by the Supreme Military Council (SMC) and backed by Western and Arab powers.
The Front has rejected the authority of the SMC, the military arm of the main political opposition in exile, and last week seized control of SMC weapons depots in northern Syria.
In a video statement given to Reuters by the SMC on Wednesday, Mr Ford described the takeover as "an extremely negative development" and urged disparate rebel groups to work together.
"If there is not tight cooperation between the different armed groups that are fighting the regime, the regime is going to be successful in surviving," he said.
"It is extremely important that the Supreme Military Council be able to play its role of coordinating. If it cannot do that, I do not see how the armed opposition can be truly effective."
Mr Kerry said on Tuesday a US meeting with the Islamic Front was possible as part of efforts to broaden opposition representation at the Geneva peace talks, but the Front's commanders are wary of openly coordinating with Washington.
Any such public engagement with the United States could put the newly formed Islamic Front on a collision course with the powerful al Qaeda-linked Islamic State in Iraq and the Levant.
The Syrian Foreign Ministry said in a statement carried on the state news agency that dialogue with the Islamic Front contradicted US and international commitments to combat terrorism, as well as "international pledges that terrorist organisations 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 Syria out of a 33-month-old conflict in which more than 100,000 people have been killed.
The Islamic Front "agrees in principle, strategy and goals with the Nusra Front", the Syrian foreign ministry said, referring to another al Qaeda-linked rebel group that the State Department last year listed as a terrorist organisation.