Washington is willing to cooperate with Moscow to end the Syrian conflict but only if there is first a "true cessation" of hostilities, US Defence Secretary Ash Carter said Thursday.

Ahead of an expected meeting in Geneva between US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, Carter told BBC radio there was "quite a long way to go" before a deal could be struck.

"If an agreement was reached it would result in a number of steps, importantly including a cessation of hostilities that could ultimately lead to greater cooperation between the United States and the Russian military," he said.

Carter added: "It is possible. However in the current circumstance, it is not possible for the United States to associate itself with -- let alone to cooperate in -- a venture that is only fuelling violence and civil war."

He said Washington wanted Assad gone "as soon as possible", adding: "We realise there'll be a transition in order to make this orderly, but that's what the Russians are supposed to be arranging."

He noted that when Russia entered the conflict in Syria, it said that it intended to fight terrorism and try to end the civil war through a political transition.

"That's not what it has done," said the defence secretary, who is in London for a UN peacekeeping conference.

"It has fuelled the civil war and the violence there and has not helped us progress to a political solution that would have (President) Bashar al-Assad step aside and have a new government including the moderate opposition take over."

He called for "a true cessation of hostilities -- not what you've seen, which is a partial cessation of hostilities", adding: "Our patience is not unlimited."

"We have our differences, serious differences, with Russia elsewhere, especially here in Europe with Ukraine and elsewhere Russia has been acting in an aggressive manner," he said.

However, he noted that Russia and the US had also worked together to secure a deal on Iran's nuclear programme.

In a speech in Oxford on Wednesday evening, Carter said the US "does not seek a cold, let alone a hot war with Russia".

But he warned: "We will counter attempts to undermine our collective security. And we will not ignore attempts to interfere with our democratic processes."

US officials have said Russia was behind an email hack that embarrassed White House hopeful Hillary Clinton, although Russian President Vladimir Putin denied this.