Madrid (AFP) - US Defence Secretary Ashton Carter said Monday that Moscow is pursuing a "losing strategy" in Syria amid rising tensions after a Russian fighter jet violated Turkey's air space.
Moscow last week began bombing in Syria, claiming it was hitting Islamic State jihadists. But the Pentagon says Russian jets have targeted other rebel groups in order to support the regime of President Bashar al-Assad.
"Russia has escalated the civil war, putting further at risk the very political resolution and preservation of Syria's structure of future governance it says that it wants," Carter said in Madrid at the start of a five-day trip to Europe.
"It remains my hope that Vladimir Putin will see that tethering Russia to a sinking ship is a losing strategy."
Carter's trip is aimed at thanking allies from the US-led coalition that is carrying out daily drone and plane strikes against IS in Iraq and Syria.
In a fresh complication of the four and a half year Syrian conflict, which has claimed around 250,000 lives, Turkey said its F-16 jets had intercepted a Russian fighter plane that violated Turkish air space near the Syrian border at the weekend.
A senior US defence official, speaking on condition of anonymity, said it was unlikely the Russian jet crossed the border by chance.
"I don't believe that this was an accident," the official said, noting that Russia had violated the sovereignty of a NATO ally.
Only last week, the Pentagon and Russia held high-profile talks on how to avoid accidents in the skies over Syria as both Moscow and the US-led coalition carry out air strikes in the war-ravaged nation.
Given the intent of those discussions, US military officials are especially alarmed about the Turkey incident.
"This is exactly the type of non-professional conduct that we were hoping to avoid," the official said.
Europe is struggling to deal with the refugee crisis sparked by the Syrian conflict, which has uprooted millions of people.
- Kunduz strike -
As part of a NATO mission, Spain in January provided a Patriot missile battery to help defend Turkey from ballistic missiles originating in Syria.
And Spain has agreed to the establishment of a permanent force of 2,200 US marines at Moron de la Frontera that Carter will visit Tuesday.
The tour comes at a fraught time for Carter, who started in the Pentagon's top job in February and who faces pressure at home over Syria and a simmering scandal involving military officials allegedly cherry-picking intelligence.
Adding to Carter's woes is the fallout from a suspected US strike that hit a Doctors Without Borders (MSF) hospital in the Afghan city of Kunduz on Saturday, killing 22 people, some of whom burned to death in their beds.
The US military has not confirmed it was responsible for the devastation, but has said it deployed an AC-130 support plane to help Afghan forces fighting near the hospital.
"We need organisations like Doctors Without Borders, and we've been in touch with them to assure them that a full and transparent investigation will be held," Carter said. "We've directed our forces in the area to make sure that all civilians who need medical care know our medical facilities are standing by to treat them."
Carter is attending a NATO ministerial summit on Thursday. Aside from the incident in Turkey, officials are expected to tackle a number of issues including Russian aggression in eastern Ukraine.
"We will continue to make it clear that if Russia wants to end its international isolation and be considered a responsible global power, it must stop its aggression in eastern Ukraine," he said.
"We will take all necessary steps to deter Russia's malign and destabilising influence, coercion, and aggression, including its efforts to undermine strategic stability and challenge the military balance in Europe."
The US is helping train Ukrainian forces and has sent "non-lethal" military aid. On the ground, the security situation in east Ukraine appears to have improved in recent weeks with a truce between government troops and pro-Russian separatists largely holding.