Recep Tayyip Erdogan, Turkey's prime minister, has told a US television station that his country will support a US-enforced no-fly zone in Syria.

In an interview with NBC News on Thursday, Erdogan said that President Bashar al-Assad's alleged use of chemical weapons against his opponents meant that the Syrian regime had already crossed US President Barack Obama's so-called red line "a long time ago".

"Right from the beginning ... we would say 'yes'," Erdogan told NBC when asked if Turkey, a NATO member that shares its longest border with Syria, would support a no-fly zone.

A no-fly zone to prohibit Syrian military aircraft from hitting rebel targets has been previously mentioned by US politicians as one of the options the US could utilise in a bid to put pressure on Assad.

But setting up a no-fly zone would require US air raids and possibly ground forces, heightening the risk of casualties.

There is little chance the US would undertake that anytime soon, US security officials have said.

Erdogan's comments are likely to add further pressure on the US to take action in a nearly two-year conflict that has already killed more than 70,000 people and further destabilised a volatile region.

Wary of the false intelligence used to justify the 2003 war in Iraq, the US says it wants proof before taking any action.

"It is clear that the regime has used chemical weapons and missiles. They used about 200 missiles, according to our intelligence," Erdogan said.

He did not make clear whether his country believed that all 200 missiles carried chemical weapons, adding his government had not determined whether sarin gas was used.

"There are different sizes missiles. And then there are deaths caused by these missiles. And there are burns, you know, serious burns and chemical reactions," Erdogan said when asked what evidence Turkey had.

"And there are patients who are brought to our hospitals who were wounded by these chemical weapons."

"You can see who is affected by chemical missiles by their burns."

Erdogan said Turkey would share intelligence with the UN Security Council in the matter.

'No evidence'

Britain said on Thursday it believed it was "very likely" that the Syrian government had used chemical arms but that it had "no evidence to date" that the rebels had done so.

Assad's forces and rebels fighters have accused each other of using chemical weapons.

Erdogan said in the interview that he doubted Assad's opponents had used such weapons because they lacked access to them.

"But if it exists, we are against this ... we are against whoever holds the weapons." he said.

Last week a UN war crimes investigator said testimony from Syrian casualties and medical staff indicated that rebels had used the banned nerve agent sarin, although other investigators later played down those suggestions.

Ahmet Davutoglu, Turkish foreign minister, criticised what he called declarations made without evidence to support them.

He had raised the issue with Jan Eliasson, UN Deputy Secretary-General, in London this week, and said that as a Syrian neighbour, Turkey had the right to know if the UN had evidence of poison gas use by rebels.