The Syrian jihadist group Fateh al-Sham Front said on Sunday that it had taken under its wing the radical Salafist Jund al-Aqsa group, recently labelled a terrorist organisation by Washington.

The announcement, on Fateh al-Sham Front's Twitter account, came as Jund al-Aqsa fighters are locked in fierce clashes with other Islamist rebel groups in northern Syria's Idlib province.

"In a bid to avoid shedding the blood of Muslims and overcome the internal fighting between us and Ahrar al-Sham, which only profits the regime, we, Jund al-Aqsa announce that we have pledged allegiance to Fateh al-Sham Front," said the online statement.

The statement was handwritten and bore the signatures of the heads of both groups.

The powerful Ahrar al-Sham group which has close ties to Fateh al-Sham Front has accused several members of Jund al-Asqa of links to the Islamic State group.

The jihadist IS is the key rival of Fateh al-Sham Front, formerly known as Al-Nusra Front and Al-Qaeda's affiliate on the battlegrounds of Syria.

Analysts said that Sunday's announcement was expected to complicate things for Fateh al-Sham Front, which changed its name from Al-Nusra Front after renouncing its ties to Al-Qaeda.

"Fateh al-Sham Front has pitched itself as a part of the opposition mainstream, but by taking Jund al-Aqsa into its fold, it has joined forces with a group that every single key opposition faction in northern Syria declared a front group for IS," said Charles Lister.

"That declaration was made as opposition groups, led by Ahrar al-Sham, were engaged in a substantial offensive to force Jund al-Aqsa out from  all opposition territories.

"So in short, Fateh al-Sham Front has illustrated that the protection of jihadists, even those so extreme as to acquire the (IS) label, are more worthy of protection than the very opposition groups it has tried so hard to embed itself within," said Lister, an analyst from the Middle East Institute.

Syrian government ally Russia has accused the United States of "protecting" Fateh al-Sham Front -- which is also considered by Washington a "terrorist organisation".

Source: AFP