Ashton Carter arrives in Europe amid Syrian, Afghan crises

The US Defence Secretary is also dealing with the fallout from a suspected US air strike on a hospital in the Afghan city of Kunduz that killed 22 people

US Defence Secretary Ashton Carter has arrived in Madrid, beginning a European tour amid alarm over Russian bombardments in Syria and the refugee crisis sparked by the conflict.

He is also dealing with the fallout from a suspected US air strike on a hospital in the Afghan city of Kunduz that killed 22 people, some of whom burned to death in their beds.

"My reaction was I think the same as anybody's," Carter told reporters on the flight to Madrid yesterday, describing the situation as "confused and complicated."

"This is a tragic loss of life, your heart can only go out to innocent people that were caught up in this type of violence."

The five-day trip to Spain, Italy, Britain and a NATO ministerial summit in Brussels is intended to recognise the work of ally nations as they scramble to tackle the refugee crisis as well as respond to Russian aggression in eastern Ukraine.

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.

The Kunduz incident adds to Carter's woes and he -- along with President Barack Obama -- has promised a full and transparent investigation.

A senior defense official said US special operations forces in an "advise and assist" role in Kunduz had been taking fire and called in air support from an AC-130. The plane opened fire but the military wasn't "positively certain" it hit the hospital, the official said.

While in Spain, Carter will visit a base in Moron de la Frontera, where a permanent force of 2,200 US marines has been established. The facility was bolstered in the wake of the 2012 attack on a US mission in Benghazi, eastern Libya.

The main mission of the Special-Purpose Marine Air-Ground Task Force Crisis Response (MAGTF) unit is to protect US embassies in Africa, evacuate civilians in difficulty, or intervene in conflicts or humanitarian crises.

Carter, 61, has been on the job for eight months but unlike some of his predecessors -- such as Donald Rumsfeld and Robert Gates -- he has not established himself as a household name.

Carter has appeared reluctant to stray from official White House talking points and critics say he is overly cautious in his public pronouncements.

The Pentagon is under massive pressure over its efforts to intervene in Syria. While the White House does not want to commit ground forces to another difficult Middle East conflict, opponents are outraged by what they see as a lack of US action to halt the war that has claimed some 250,000 lives and displaced millions.