US Secretary of State John Kerry on Monday appealed to the Syrian opposition to reconsider their decision to boycott talks with foreign powers in Rome this week.
"I would urge the Syrian opposition to join us," Kerry told a news conference in London after talks with British Foreign Secretary William Hague at the start of his first overseas trip since taking office.
"I want our friends in the Syrian opposition council to know that we are not coming to Rome simply to talk. We are coming to Rome to make a decision on next steps," he said.
He stressed however that the US continued to pursue a political resolution to a conflict which the United Nations believes has cost the lives of more than 70,000 people.
The Syrian National Council has said it will pull out of the Friends of Syria talks in the Italian capital on Thursday in protest at the international community's "shameful" inability to halt the bloodshed.
But Kerry argued: "We think it's important to get together to hear directly from the opposition to know precisely what they think would be most useful at this point in time, how we can make a difference.
"I think it's an important meeting... and I still remain hopeful they will make the decision to come and join us."
Kerry, who is conducting a nine-country tour of US allies, said the killings in the Syrian city of Aleppo last week were "further evidence" that President Bashar al-Assad must stand down.
"The Syrian people deserve better than the horrific violence that now invades and threatens their everyday lives," Kerry said.
--- 'Hard to see how regime ready for talks' ---
He noted that Assad's foreign minister Walid al-Muallem said in Moscow on Monday that 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.
Kerry said: "It seems to me that it's pretty hard to understand how, when you see these Scuds (missiles) falling on the innocent people of Aleppo, it's possible to take their notion that they're ready to have a dialogue very seriously."
National Council chief Ahmed Moaz al-Khatib said on Saturday the opposition was withdrawing from the Rome meeting as "a message of protest to all governments of the world, Arab and non-Arab, that can see how the Syrian people are being killed, while they merely look on."
A State Department official said Kerry had spoken to Khatib by phone "to encourage him to come to Rome".
Hague, who is due to attend Thursday's talks, said Britain shared the frustrations about inaction on Syria.
In the last two years "there has been no sign of a political and diplomatic breakthrough," he said. "Our frustration is intense as well."
Hague added: "In the face of such murder and threat of instability, our policy cannot remain static as the weeks go by, and it is an important opportunity in Rome on Thursday to discuss this with our allies and partners."
Britain wants to provide more support for the Syrian rebels but is bound by an EU arms embargo, which European foreign ministers decided against lifting at a meeting last week.
Kerry goes on from London to Berlin where he will hold talks on Tuesday with his Russian counterpart Sergei Lavrov, when he is expected to push Moscow to put pressure on the Syrian regime to allow a "political transition".
Russia is one of the few major powers who still maintain ties with Assad's regime.
Kerry and Hague also discussed Iran's nuclear ambitions, the day before the resumption of talks over a programme that Tehran insists is for civilian purposes but world powers fear is intended to build a nuclear weapon.
Kerry warned that the window for a diplomatic solution would not be open forever, adding: "It is open now and there is still time, but there is only time if Iran makes the decision to come to the table and negotiate in good faith."
His get-acquainted tour, the first since taking over from Hillary Clinton last month, will also include stops in France, Italy, Turkey, Egypt, Saudi Arabia, the United Arab Emirates and Qatar.