But there have been no contacts with groups such as Al-Nusra that are blacklisted by Washington as terror organizations, deputy State Department spokeswoman Marie Harf stressed on Wednesday.
"We engage with a broad cross-section of Syrian people and political and military leaders in the opposition, including a variety of Islamist groups. We do not engage with terrorists, with groups that are designated as terrorist organizations," she told reporters.
The Islamic Front is now Syria's largest armed opposition, grouping tens of thousands of fighters battling to oust President Bashar al-Assad in the nearly three-year war.
The alliance has said that it wants to create an "Islamic state," but insisted it would protect minorities and not create an "oppressive, authoritarian system."
More secular groups, such as the Syrian Free Army under the command of General Selim Idriss, have been losing ground as they battle both Assad's forces and separate militias linked to Al-Qaeda.
The goal of the US diplomatic outreach was to persuade some of the militants to back peace talks due to be held in Geneva on January 22, Western officials told a US daily.
Harf said that the talks were "in response to a reality that the opposition is made up of a number of groups, some are Islamist groups, and that in order to get a political solution here, because there is no military solution, we need to get these groups to buy into the notion that there should be a (political) solution."
She refused to specify, however, with which groups the United States was speaking, and stressed that US assistance was still only going to the Supreme Military Council led by Idriss.
The Syrian opposition has yet to determine who would be included in its delegation to the Geneva talks.     
However, Harf agreed that the "overall goal" was to get a broad section of the opposition to sit down at the negotiating table.


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