NATO's secretary general has said a "substantial" build-up of Russian forces in Syria includes ground troops.

Although Vladimir Putin has said he would not put troops in Syria, Russian military chiefs have raised the prospect of "volunteer" soldiers joining the fighting.

If employed, the tactic would have echoes of the crisis in Ukraine, where Russian troops apparently helped rebels while Moscow denied its soldiers were involved in the conflict.

Jens Stoltenberg also said he doubted Russia's claim that violations of Turkey's airspace were a mistake because there were two incursions and they lasted longer than a few seconds.

He said the incidents were "very serious" and added: "It doesn't look like an accident, and we've seen two of them over the weekend."

The Russian government is reportedly looking into the second alleged violation of Turkish airspace, after the Kremlin's ambassador was once again summoned in Ankara for an explanation.

Moscow has said the Syrian airbase from which Russian planes were flying missions, Khmeimim, was about 19 miles (30km) from the Turkish border and that its aircraft had to approach it from the north in certain weather conditions.

Russia has faced international criticism over its decision to launch airstrikes in Syria, in part because they are viewed by the West as undermining the US-led campaign.

A US official, speaking on the condition of anonymity, said Moscow had agreed on Tuesday to resume talks with the United States aimed at settling rules for air-to-air conduct over Syria.

The talks have gained urgency in recent days, because of the prospect of direct contact between US and Russian warplanes flying uncoordinated missions over Syria.

No dates for the talks have yet been set.

Meanwhile, UK Prime Minister David Cameron has acknowledged the need to set out a clearer end goal for Syria should the issue again be debated in Westminster.

“We need to demonstrate that we have a clear vision and a plan for what Syria should look like, and I’m clear that means a government not led by Bashar al-Assad, but led by someone new who can bring the country together," Mr Cameron told Sky News.

"We need a Syrian government that can appeal to the Alawites, but also to the Sunnis, the Kurds and Christians."

He said it was “in the national interest” to destroy Islamic State.