BEIRUT – Syria’s increasingly powerful Islamist rebel factions rejected the country’s new Western-backed opposition coalition and unilaterally declared an Islamic state in the key battleground of Aleppo, a sign of the seemingly intractable splits among those fighting to topple President Bashar Assad.

The move highlights the struggle over the direction of the rebellion at a time when the opposition is trying to gain the West’s trust and secure a flow of weapons to fight the regime. The rising profile of the extremist faction among the rebels could doom those efforts.

Such divisions have hobbled the opposition over the course of the uprising, which has descended into a bloody civil war that has seen nearly 40,000 deaths. The fighting has been particularly extreme in Aleppo, Syria’s largest city and a major front in the civil war since the summer.

Salman Shaikh, director of The Brookings Doha Center in Qatar, said Monday the Islamists’ declaration will unsettle both Western backers of the Syrian opposition and groups inside Syria, ranging from secularists to the Christian minority.

“They have to feel that the future of their country could be slipping away,” Shaikh said. “This is a sign of things to come the longer this goes on. The Islamist groups and extremists will increasingly be forging alliances and taking matters into their own hands.”

The West is particularly concerned about sending weapons to rebels for fear they could end up in extremists’ hands.