Spokesman says Cameron hasn't ruled anything out after Hammond insisted that UK won't be taking part in strikes

David Cameron has overruled the foreign secretary Philip Hammond after he appeared to rule out UK involvement in air strikes against Islamic State (Isis) forces in Syria.

Hammond made his remarks at a press conference in Berlin, but Cameron's spokesman later said that as far as the use of British air power was concerned Cameron had not ruled anything out. The spokesman said he was clarifying Hammond's remarks, insisting the foreign secretary was referring to the possibility of air strikes against Bashar al-Assad.

Asked by Reuters after his meeting with German foreign minister Frank-Walter Steinmeier about Barack Obama's proposal for air strikes against Isis in Syria, Hammond replied: "Let me be clear: Britain will not be taking part in any air strikes in Syria. We have already had that discussion in our parliament last year and we won't be revisiting that position."

Hammond either made a mistake or as likely Number 10 did not want Hammond to suggest at this stage there was any difference in policy between London and Washington. His remarks contrast with those of the US president, who said on Wednesday that the US military would be able to identify targets in Syria capable of being bombed from the air.

On Thursday ten Arab states released a statement in which they agreed to rally behind the US in tackling the Isis threat.

Russia's foreign ministry earlier said that air strikes on Isis in Syria without a UN security council mandate would be an act of aggression.

"The US president has spoken directly about the possibility of strikes by the US armed forces against Isis positions in Syria without the consent of the legitimate government," ministry spokesman Alexander Lukashevich said. "This step, in the absence of a UN security council decision, would be an act of aggression, a gross violation of international law."

The British government considers the issue of air strikes in Syria to have been discussed by MPs last year and does not want to revisit it.

Air strikes in Syria would face legal difficulties because, while the government in Iraq is seeking formal support from an international alliance to help to drive Isis from its territory, no such request will come from Assad.

Hammond told MPs on Wednesday: "There is a qualitative difference between any proposition of air strikes in Syria and such an activity in Iraq. The legal, technical and military differences make the proposition of air strikes an order of magnitude more complicated in Syria."

He also referred to the military difference between the two countries: "In Iraq, the skies are open over Isis-controlled territory, whereas in Syria a sophisticated, integrated air defence system protects the whole of the country's airspace and would make air strikes complex and difficult to deliver."

He said that for the moment the government had not decided whether to arm Syrian moderate rebels.

At one point David Cameron suggested the UK would not legally need a request from the Syrian government to strike against Isis there, arguing that the Assad government was not legally recognised due to its involvement with war crimes.

The prime minister has rejected suggestions, notably from the former foreign secretary Jack Straw, that the government should end its efforts to dislodge Assad, and recognise that the chief goal must be to form an alliance to drive Isis out of Syria and Iraq. Cameron insisted he thought it was still possible that the moderate Syrian rebels could be helped to improve their position against both Assad and Isis.