British Foreign Secretary Philip Hammond and US Secretary of State John Kerry held talks showing different attitudes towards Russia's involvement in the Syria crisis.
After the meeting at Hammond's official residence on Saturday, Kerry said defence secretaries of the US and Russia had held extensive talks via telephone on the fight against the IS, reported Xinhua news agency.
Kerry, who is on his three-day visit to Britain, said "Would we welcome Russian help in going against IS? Obviously. We have talked about it for some period of time."
"But the other part of the equation is Assad (President of Syria) and how you resolve the fact that he is a magnet for foreign fighters to come to the region," he said.
The British government had been clear that if it is believed airstrikes in Syria were necessary it would go back to parliament for a vote, Hammond said.
"We keep that situation under continuous review and we will continue to talk to our allies about the military situation in the round," he said.
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Hammond said the situation in Syria becomes more complicated because of the Russian engagement.
"I think we need to discuss this as part of a much bigger problem: the migration pressures, the humanitarian crisis in Syria as well as the need to defeat IS," he added.
Kerry described Russia's new focus on fighting IS militants as an opportunity to push toward a political settlement.
Kerry said the "root cause" of the current migrant crisis in Europe was the violence in Syria.
He said the US is prepared to negotiate, but have not known if Syrian President Bashar al-Assad and Russia were prepared to a negotiation.
The US has called for Assad to go, but on Saturday Kerry reaffirmed the US position that this removal "does not have to be on day one or month one... There is a process by which all the parties have to come together to reach an understanding of how this can best be achieved," according to BBC.
He said the talks in London laid the groundwork for forthcoming meetings at the UN over Syria. He will leave for Berlin on Sunday.