Syria's domestic opposition on Monday snubbed a call to dialogue from President Bashar al-Assad, as NATO powers challenged the leader's defiant grip on power and dispatched Patriot missile batteries to neighbouring Turkey.

The cold shoulder by the National Coordination Body (NCB) for Democratic Change in Syria, a tolerated non-armed opposition group separate from the internationally recognised armed opposition Syrian National Coalition, left Assad's proposal without any takers.

And the common front by the United States, France, Britain and Turkey accentuated the international isolation Assad is suffering, a day after he gave a speech declaring he would not stop pounding "terrorist" rebels.

In the speech, Assad urged talks with opposition elements he deemed acceptable, excluding the main exiled opposition as "killers" dominated by non-Syrians and working for foreign masters.

But the head of the NCB, Hassan Abdel Azim, told a news conference in Damascus his group "will not take part in a national dialogue before violence stops," political prisoners were released, humanitarian aid was delivered to conflict-torn areas, and missing Syrians were highlighted in a statement.

He also stated "there won't be direct negotiations or dialogue with the regime," but only within a framework headed by joint UN-Arab League peace envoy Lakhdar Brahimi.

UN leader Ban Ki-moon said in a Monday statement Assad's speech would not end the Syrian people's "terrible suffering."

Ban said he and Brahimi believed a "political transition that includes the establishment of a transitional government and the holding of free and fair (UN-monitored) elections" was necessary.

Washington and Paris said Assad had lost touch with reality, while London branded as "empty" the Syrian leader's promise to hold dialogue.

Writing on Twitter, French Foreign Minister Laurent Fabius said: "We knew that Bashar was a murderer of his people, we can hear in listening to his appalling speech that he is also deaf and blind."

Turkish Prime Minister Recep Tayyip Erdogan said Assad was guilty of "state terrorism" by pursuing a civil war that has killed more than 60,000 people since it started 21 months ago, according to the UN.

"He is shifting the blame by designating his opponents as Al-Qaeda or terrorists," Erdogan said.

Dutch Patriot missile batteries left for Turkey's border with Syria to join US and German batteries. Some 400 US troops are already arriving in readiness.

The deployments have been strongly criticised by Syria's main ally Iran, even as the Islamic republic was believed to keep up a flow of money, military advisers and, according to the US, weapons to Assad's regime.

Tehran said it supported Assad's "comprehensive" plan to restore peace but made no mention of its own six-point "peace plan" for Syria which has been largely ignored internationally.

The Syrian National Coalition said Assad's declaration ruling out any dialogue with the rebels and his insistence on staying in power made any negotiations impossible.

Analysts said it appeared Assad thought himself to have enough of a military advantage to make such an uncompromising address.

"Today, the situation is stabilising a bit. The regime won't win the war, but it knows it will still go on for some time," said Thomas Pierret, Syria expert and professor at the University of Edinburgh.

"All of these developments are giving breathing space to a regime doomed in the future, which explains the timing of the speech," he said.

On the ground, fighting raged in the northern city of Aleppo where 12 people were killed in army shelling, a watchdog said.

The Syrian Observatory for Human Rights gave an initial toll of 50 killed nationwide on Monday, including 34 civilians.