Al-Qaeda-linked Al-Nusra Front has issued a new threatening audio message featuring its leader warning the West “will pay the heaviest price” for its actions. The Syrian group is reportedly now joining up with the estranged Islamic State militants.

The leader of Syria’s most prominent terrorist group, Abu Mohamad al-Golani, in denouncing the US-led air strike campaign, has urged Westerners everywhere to do the same “by standing against the decisions of your rulers,” otherwise bloodshed would be brought to their soil.

"Muslims will not watch while their sons are bombed. Your leaders will not be the only ones who would pay the price of the war. You will pay the heaviest price," Reuters cited him as saying. He threatened viewers that the fight would be brought “to the hearts of your homes.”

The US-led coalition has been involved in airstrikes against what until lately it thought was the most dangerous group in the Middle East – the Islamic State (IS, formerly ISIS/ISIL).

However, recent intelligence has pointed to the danger of avoiding other groups whose modus operandi involves carrying out attacks on American and European targets. The IS’s so far has not.

READ MORE: US admits there is a much scarier terrorist group than ISIS

The US has opened two airstrike fronts in its war against the IS: Iraq, since August 8, and Syria since September 23.

The promise was to “degrade and destroy” the terrorist group, while Al-Nusra Front and Al-Qaeda were reduced in importance.

Indeed, for the past year, the IS had fallen out of favor with the likes of Ayman al-Zawahiri, the Al-Qaeda leader who tried to keep the militants in check. The reason was the group’s unwillingness to operate only in Iraq.

Now, however, the American airstrike campaign appears to have brought the two back together again: Al-Nusra Front has come under pressure from its own members to make good with the IS and embark on a mission to repel the "crusader" assault on Islam.

Although the two groups had fought a bitter battle on the sidelines of the broader Syrian conflict against President Bashar Assad’s forces, a senior Al-Nusra Front source has confirmed to the Guardian that a series of war planning meetings is underway.

While there’s still no word of a deal, any potential unity could be seen as a reason to worry. The Western airstrike campaign has been aiming to cripple the IS’s funding sources in order to slow its progress in Syria and Iraq. The addition of at least some elements of the Syria-based Al-Nusra Front to IS ranks would be a counter-balancing factor. In fact, 73 members had already reportedly defected to IS last Friday, according to an Al-Nusra Front source speaking to the Guardian.

The official Al-Nusra Front spokesman paraphrased in another message the earlier words of Al-Qaeda leader Zawahiri, that this is now a full-on “war.” And as Zawahiri said, “this war will not end in months nor years, this war could last for decades.”

READ MORE: Retaliation for US-led airstrikes in Syria will follow, Al-Qaeda offshoot vows

With the Islamic State rampaging across northern Syria and Iraq during the past year, Al-Nusra Front was in relative obscurity. In Tuesday’s strikes by the US, however, 50 of its fighters were killed, including the leader of Al-Nusra-linked Khorasan – the group reportedly tasked with carrying out Al-Nusra’s attacks abroad.

Gholani made sure to mention that losses by all said groups make an imprint on the entire campaign, and will provoke retaliation, adding that in the end “even if we suffer some pain during it,” the war will be won.

Most importantly, he urged all Middle Eastern groups who had suffered at the hands of the IS to not use the opportunity to strike back at them, and instead unite to fight the West.

"[IS injustice] should not push any of you to be driven behind the West and take part in the alliance which they want to use to end jihad," he said.

He further appealed to Sunni Muslims in Lebanon to leave the army and rise up against Hezbollah – the Shiite militia – in order to also fight the Shiite-aligned Assad in Syria.

Some Islamist elements of Syria’s three-year-long opposition are also visibly angry that the airstrike campaign is going nothing to offset the gain of the Syrian government.

“We have been calling for these sorts of attacks for three years and when they finally come they don’t help us,” said the leader of the Qatar-sponsored Islamic Front.

Airstrikes, however, also kill civilians. One such strike killed 31 civilians, when a school near the Iraqi city of Tikrit was hit on September 1. This included 24 children and a further 41 wounded civilians.

In the meantime, US President Barack Obama has in a CBS interview blamed intelligence for “underestimating” the threat posed by Islamic State in Syria, adding also that the fall of Iraq’s army in the north was likewise unexpected and is allowing the terrorist group to “reconstitute themselves and take advantage of that chaos.”