NATO General Secretary Jens Stoltenberg has told a Swiss newspaper,  the Tages-Anzeiger, that NATO putting boots on the ground in Syria to fight Islamic State (Isis) militants is out of the question.

"That is not on the agenda of the coalition and the NATO allies," he told the newspaper when asked about dispatching ground forces to accompany air strikes, according to a Reuters report.

Instead, the need of the hour is to bolster local forces in the conflict, he said.

"The United States has a limited number of special forces. In the foreground, however, is strengthening local forces. This is not easy, but it's the only option," he told the newspaper, adding that the situation in Syria was not a war between the West and the Islamic world, but rather a war against "extremism and terrorism".

"Muslims are on the front line in this war. Most victims are Muslims, and most of those who fight against the IS are Muslims. We can not carry on this struggle for them," he said, according to Reuters.

Stoltenberg confirmed that Turkish air defenses would be strengthened by NATO in the wake of the NATO ally shooting down a Russian fighter plane last month. Before Christmas, more defensive measures would be in place, he claimed. The move comes in the wake of Russia deploying powerful S-300 missiles in Syria after the loss of its fighter jet.

However, the NATO chief said "Now it is important to de-escalate and to develop mechanisms to prevent similar incidents in the future. We see a significant build-up of the Russian military presence from the far north to the Mediterranean. There, too, we need to avoid similar incidents such as in Turkey."

He criticised Russia for bolstering the Assad regime in Syria and claimed that Russia had been attacking other Syrian groups instead of joining a broad coalition in the fight against Isis.

Meanwhile, U.S. President Barack Obama has clarified that the decision to send in special forces to Iraq did not mean that the US was about to get involved in an invasion as in 2003. Fighting Isis militants does not include US boots on the ground, he reiterated.