Al-Qaeda's leader in Iraq has rejected orders from Ayman al-Zawahiri, the group's global chief, to break up his group's claimed union with the Jabhat al-Nusra, an armed Islamist group in Syria, according to a new audio message.

The purported remarks by head of Islamic State of Iraq (ISI) Abu Bakr al-Baghdadi in the message posted on jihadist forums on Saturday indicate tensions between ISI and al-Qaeda's central command.

In April, Baghdadi announced that ISI had merged with Syria's Jabhat al-Nusra, or al-Nusra Front.

Al-Nusra leader Abu Mohammed al-Jawlani acknowledged a relationship between the two groups, but he denied there had been a merger and publicly pledged his allegiance to Zawahiri.

In Saturday's message, the man identified as Baghdadi said "the Islamic State of Iraq and the Levant will remain, as long as we have a vein pumping or an eye blinking. It remains and we will not compromise nor give it up".

"It remains, and we will not compromise; we will not give up [...] until we die."

Earlier this month, Zawahiri ruled that the ISI and al-Nusra should operate as separate entities, according to a letter released to Al Jazeera.

Baghdadi had "made a mistake" by announcing a merger "without consulting us", he said.

The merger plan has been "damaging to all jihadists", Zawahiri said, adding that "Al-Nusra Front is an independent branch of Al-Qaeda".

But the message on Saturday said: "When it comes to the letter of Sheikh Ayman al-Zawahiri - may God protect him - we have many legal and methodological reservations."

After consulting with the consultative council of the Islamic State of Iraq and the Levant [...] I chose the order of God over the orders that contravenes Allah in the letter.

The audio message could not immediately be independently verified.

Al-Nusra Front, created in January 2012, joined al-Qaeda last December on a US list of foreign terrorist organisations.

An al-Nusra front member in Syria, speaking on condition of anonymity, told Al Jazeera that following the release of Zawahiri's letter, many members of ISI rejoined al-Nusra, particularly in the province of Deir Ezzor.

He said this new audio recording causes further division and confusion among those fighting on the ground.

"Defying the orders of Zawahiri is a black dot on Baghdadi's career", he said.

Among elements fighting to oust the regime of Syrian President Bashar al-Assad, al-Nusra is one of the best armed and most successful on the battlefield. It has carried out some of the deadliest attacks in the uprising, claiming responsibility for several suicide bombings.

Additional reporting by Al Jazeera's Basma Atassi.

Source: Al Jazeera and agencies