New US Secretary of State John Kerry met British Prime Minister David Cameron on Monday at the start of a get-acquainted tour of Europe and the Middle East, but the Syrian conflict threatened to taint the trip.
US officials were trying to persuade the Syrian opposition to reconsider its threat to boycott an international meeting in Rome on Thursday, which was to be the centrepiece of Kerry's first overseas tour since he took over from Hillary Clinton.
The Syrian National Council has said it will pull out of the 11-nation Friends of Syria talks in protest at the international community's inability to halt the bloodshed.
Kerry is due to meet his Russian counterpart Sergei Lavrov in Berlin on Tuesday and is expected to push Moscow to put pressure on the Syrian regime to allow a "political transition".
Lavrov held talks in Moscow on Monday with President Bashar al-Assad's foreign minister, who said the Syrian regime was ready for talks with the rebels and anyone who favours dialogue, in the first such offer by a top Syrian official.
Russia is one of the few major powers who still maintain ties with Assad's regime.
A senior US official said on Sunday: "We feel that Russia can play a key role in convincing the (Syrian) regime that there is need for political transition."
Kerry, a former US presidential candidate, made a gentle start to his marathon tour of close US allies by enjoying a traditional English breakfast with Cameron at Downing Street on Monday.
The two men discussed Syria and also talked of how to achieve a trade deal between the United States and the European Union.
They agreed that the G8 summit in Northern Ireland in June "will be an important moment to inject further momentum into this", according to a statement from Cameron's office.
Kerry then went into talks with British Foreign Secretary William Hague which will be followed by a joint news conference later before Kerry heads to the German capital.
The trip will also include stops in France, Italy, Turkey, Egypt, Saudi Arabia, the United Arab Emirates and Qatar.
Kerry is also expected to address issues including Iran, Mali and North Korea during the two-week tour.
US officials were privately optimistic that they could persuade the Syrian opposition to attend the talks on Thursday.
Publicly, a State Department official said: "We are stressing... that they (the opposition) have an opportunity in Rome, to see the countries that have been their greatest supporters and to present to all of us how they see the situation on the ground in security, humanitarian, political and economic terms.
"This meeting is also an opportunity for them to meet our new secretary of state and to speak directly to him," he added.
Syria will also dominate Kerry's talks in Ankara and Cairo, where he is due to meet with the Secretary General of the Arab League, Nabil al-Arabi.
The Kerry-Lavrov meeting on Tuesday will take place at the same time as talks in Kazakhstan between the so-called 5+1 world powers -- US, China, Russia, France, Britain and Germany -- and Iran over the issue of Tehran's nuclear policy.
A US official said strengthened sanctions were having a "real effect" in Iran and welcomed the "common" position among the group.
The trip sees Kerry, the son of a diplomat, back on familiar ground. He spent part of his childhood in Berlin and has family in France.