U.S. Secretary of State Hillary Clinton has warned Syrian opposition forces against allowing the incursion of extremist groups into their ranks.

While on a trip to Croatia, Clinton said the rebels should “strongly resist the efforts by the extremists to hijack the Syrian revolution."

The Syrian opposition is a loosely organized group of rebel factions, which agree on armed rebellion to remove President Bashar al-Assad from office but do not always agree on tactics.

Over the course of the 19-month-old conflict, there have been increasing reports of Islamist militant groups entering the country and fighting alongside the rebels, while seeking to establish their influence in the region, according to the BBC.

The Al-Nusra Front, a paramilitary group affiliated with al Qaeda, has been active in Syria and claimed responsibility for bomb attacks and is suspected of setting off car bombs during the four-day cease-fire agreement last week. That pact quickly unraveled as the Syrian military responded with heavy shelling and airstrikes.

Clinton also emphasized the need for more representative leadership among the rebels.

"There has to be a representation of those who are in the front lines fighting and dying," Clinton said.

"This cannot be an opposition represented by people who have many good attributes but who, in many instances, have not been inside Syria for 20, 30, 40 years."

Clinton was making reference to the Turkey-based Syrian National Council, a coalition of political opposition groups that have been planning to step in and set up a new government should Assad be removed.

It has attempted to negotiate a diplomatic solution to the conflict but failed to stop the violence, while having little representation of rebel leaders that have been fighting on the ground.

Opposition groups are expected to meet in Qatar next week, a parley which Clinton hopes will produce a cohesive strategy on ending the Syrian conflict that has already claimed at least 20,000 lives.