Six armed opposition factions in Syria have announced that they are joining the ranks of Ahrar al-Sham, one of the country's largest rebel groups.

Their decision came after Jabhat Fateh al-Sham, formerly known as Jabhat al-Nusra, attacked their positions in Idlib and Aleppo provinces. The six factions include Alwiyat Suqour al-Sham, Kataib Thawar al-Sham, Jaish al-Mujahideen and Tajamo Fastaqim Kama Umirat, along with Jaish al-Islam's Idlib branch and al-Jabha al-Shamiya's west Aleppo branch.

Ahrar al-Sham issued a statement on Thursday welcoming the factions and warning that any attack on them would be considered "a declaration of war".

Tensions between Jabhat Fateh al-Sham and armed opposition groups, including the Free Syrian Army (FSA), escalated on Tuesday evening after Jabhat Fateh al-Sham attacked factional headquarters across the two provinces.

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A commander of one FSA faction, who spoke on condition of anonymity, told Al Jazeera that Jabhat Fateh al-Sham continued to attack the six armed groups, even after the announcement. According to him, sporadic clashes occurred between Ahrar al-Sham and Jabhat Fateh al-Sham throughout the day.

He also said that Jabhat Fateh al-Sham had attacked FSA positions in Idlib province and captured a number of its fighters, but cited no casualties in the clashes.

He denied media reports that Jabhat Fateh al-Sham had wiped out a number of FSA factions, but confirmed that the group had captured the headquarters of Jaish al-Mujahideen, Jaish al-Islam and al-Jabha al-Shamiya.

The commander told Al Jazeera that the situation remained tense in Idlib province and that Ahrar al-Sham and a number of senior religious figures were involved in negotiations to restore calm.

Nusra is trying to present it as though the FSA factions want to surrender, to have the Assad regime stay in power and reach a settlement with Russia. Hamza al-Mustafa, researcher at the Arab Centre for Research and Policy Studies in Doha

In an earlier statement, Ahrar al-Sham had said that it was sending its forces to prevent Jabhat Fateh al-Sham attacking or committing injustices against other factions.

Ahrar al-Sham was among the seven armed groups that Russia declared as "moderate opposition", which were part of the ceasefire announced on December 30, 2016. However, Ahrar al-Sham opted not to participate in the recent Astana talks, while the FSA, Jaish al-Islam and other armed opposition groups sent a delegation.

"Nusra is trying to present it as though the FSA factions want to surrender, to have the Assad regime stay in power and reach a settlement with Russia. By attacking them, it supposedly is preventing such a settlement from taking place," Hamza al-Mustafa, a researcher at the Arab Centre for Research and Policy Studies in Doha, told Al Jazeera.

Mustafa said that Jabhat Fateh al-Sham's attacks on the FSA and other armed opposition groups were prompted by the group's desire to prevent a military union between the FSA, Ahrar al-Sham and various other rebel factions. The current standoff has its roots in a long-term rivalry between the two groups, and in the refusal of Ahrar al-Sham to join Jabhat Fateh al-Sham in establishing an Islamic emirate, Mustafa said.

The FSA commander interviewed by Al Jazeera said that his faction was attacked previously by Jabhat Fateh al-Sham in 2015, before the group changed its name and severed ties with al-Qaeda. He said that he believes the group is attacking the FSA in an effort to establish an Islamic emirate.

Both Jabhat Fateh al-Sham and Ahrar al-Sham have suffered from internal divisions as a result of the current standoff, Mustafa noted.

Jabhat Fateh al-Sham leader Abu Mariya al-Qahtani's condemnation of the clashes - along with the refusal of some of his fighters to carry out orders to attack the FSA, and the defection of others to Ahrar al-Sham - has pressured Jabhat Fateh al-Sham to temporarily halt its attacks, he added.

Follow Mariya Petkova on Twitter: @mkpetkova