Moscow has said it will work to avert potential mishaps between Russian and US pilots flying missions over Syria, Pentagon officials said Tuesday.

US and Russian officials held discussions last week -- at Russia's request -- on establishing measures to avoid accidents so warplanes flying over Syria would not be in the same place at the same time.

The so-called "deconfliction" talks came after Russia started bombing in support of Syrian President Bashar al-Assad, further complicating the four-and-a-half-year conflict.

But despite Russian violations of Turkish air space at the weekend, Moscow has not participated in further talks -- frustrating US military officials who'd made repeated overtures.

US Defence Secretary Ashton Carter has spoken repeatedly in recent days about Russia's "irresponsible and unprofessional" moves in Syria.

Moscow on Tuesday seemed to respond to those remarks.

A senior defense official, speaking during a European tour by Carter, said Moscow had indicated it was open "in principle" to carrying out pledges made during the first round of talks.

Officials have said these commitments could include undertakings on which language Russian and American pilots will use for communication, the choice of radio frequency for distress calls and the altitude at which warplanes will operate.

"We look forward to the formal response from the Russians and learning the details. We stand ready to meet again to continue our earlier discussion as soon as possible," Pentagon press secretary Peter Cook said.

Turkey says Russian fighter jets violated its air space near the Syrian border on Saturday and Sunday, further heightening tensions.

On Tuesday, it said that eight Turkish F-16 jets, carrying out reconnaissance flights over the Turkish-Syrian border a day earlier, were held on radar lock by an unidentified MiG-29.

Radar lock enables a warplane's missile systems to automatically follow a target.

- 'Strengthen our posture' -

Carter said violations of Turkish airspace would "cause us further to strengthen our posture with respect to Russia," although he did not elaborate.

The Pentagon has repeatedly stressed Russia's involvement in Syria would not alter continued air strikes against Daesh jihadists there and in Iraq as part of a coalition of more than 60 nations.

Carter could not confirm reports Moscow was committing "volunteer" ground troops to fight in Syria, but said if true, "that would simply be deepening their mistake".

Russia already has at least 2,000 military personnel in Syria, Pentagon officials say. They are stationed at an air base in the Latakia region in the country's northwest.

A defense official told AFP that Russia had moved artillery and helicopters to support potential operations in the areas of Latakia, Homs and Mara.

Carter visited Sigonella, a US-Italian base in Sicily, after stopping at another base in Moron de la Frontera in southern Spain, where Washington and Madrid have agreed the permanent establishment of a force of 2,200 US marines.

Carter's five-day trip to Spain, Italy, Britain and a NATO ministerial meeting in Brussels is intended to recognise the work of ally nations as they scramble to tackle the refugee crisis sparked by Syria and react to new threats on NATO's "Southern Flank," including unrest in North Africa.

The tour comes at a fraught time for Carter, who started in the Pentagon's top job in February.

He is dealing with the fallout from a US air strike on a hospital in the Afghan city of Kunduz that killed 22 people.

Carter's top general in Afghanistan earlier acknowledged the strike had been in error.

"The Department of Defense deeply regrets the loss of innocent lives that resulted from this tragic event," Carter said.

"The US military takes the greatest care in our operations to prevent the loss of innocent life, and when we make mistakes, we own up to them," he added.

Doctors without Borders, the medical charity known by its French acronym MSF, which was operating in the hospital when it was struck, has called the incident a war crime.

Mego Terzian, president of MSF France, rejected the claim the attack was an error.

"Unfortunately, I am even certain that it was not a mistake," Terzian said during a hearing in Paris.

Carter meets with leaders in Rome on Wednesday, including Italy's president and defence minister.
Source: AFP