John Kerry has urged Syria's opposition to attend an international meeting in Rome this Thursday, on his first trip abroad as US Secretary of State.
Speaking in London, Mr Kerry said he understood Syrians wanted results from the summit and promised it would not just be a talking shop.
He has held talks with PM David Cameron and Foreign Secretary William Hague.
Syria's opposition has said it will not attend the Rome talks because of "the world's silence" over Syrian violence.
After his talks in London Mr Kerry had a telephone discussion with Syrian Opposition Council President Moaz Al-Khatib, encouraging him to attend the Rome talks, but no more details of the conversation were released.
Mr Hague said the UK was preparing to "significantly increase" its support for Syria's opposition.
Mr Kerry, who succeeded Hillary Clinton, is on an 11-day tour of Europe and the Middle East, and will meet Syrian opposition members and Russian Foreign Minister Sergei Lavrov in Berlin on Tuesday.
"The Assad regime has rained down rockets on the people of Aleppo" in the last few days, said Mr Kerry, as he condemned the "indiscriminate killing of civilians".
He said such attacks were further evidence that Syrian President Bashar al-Assad should go.
Mr Hague said 70,000 people had been killed and he was "frustrated" at the lack of an international political solution, despite efforts at the UN.
Earlier, the Syrian foreign minister said his government was ready for talks with its opponents, even armed rebels.
Mr Kerry said the US would continue with its policy of not taking a position on the sovereignty of the Falkland Islands.
Argentina claims the territory - which it calls the Malvinas - and says it will not recognise a forthcoming referendum organised by the islanders on how they want to be governed.
Mr Kerry said he would not comment on the referendum.
"The United States recognises de facto UK administration of the islands, but takes no position on the question of party's sovereignty claims thereto," he said. "And we support co-operation between UK and Argentina on practical matters and we continue to urge a peaceful resolution of that critical issue."
A Downing Street spokesman said Mr Cameron and Mr Kerry "reiterated their shared determination to prevent a nuclear-armed Iran" and discussed "the challenges posed by fragile states around the world".
The top US diplomat said Britain and the US remained implacably opposed to the prospect of Tehran obtaining nuclear weapons, saying an Iran with a nuclear weapon was "simply unacceptable".
"The window for a diplomatic solution simply cannot by definition remain open forever," he told a news conference. "But it is open today.
"There is still time but there is only time if Iran makes the decision to come to the table and negotiate in good faith. We are prepared to negotiate in good faith, in mutual respect in an effort to avoid whatever terrible consequences could follow failure.
"The choice really is in the hands of the Iranians. And we hope they will make the right choice," said Mr Kerry.
The prime minister's spokesman said Mr Kerry did not ask about Britain's possible future exit from the European Union but he and Mr Cameron did discuss a free-trade agreement between the US and the EU and the possibility of making progress at the next G8 summit in Northern Ireland in June.
In all, Mr Kerry will visit nine countries, stopping off in Paris, Berlin, Rome, Ankara, Cairo, Riyadh, Abu Dhabi and Doha.
The British government is pleased that Mr Kerry made London his initial port of call on his first official trip abroad, says BBC diplomatic correspondent Bridget Kendall.
His itinerary stands in marked contrast to his predecessor, Hillary Clinton, who rather pointedly went to Asia first, our correspondent says.
Though the Middle East peace process will be on the agenda when he visits Turkey, Egypt and Saudi Arabia, Mr Kerry will not visit Israel or the Palestinian Territories.
However, our correspondent says Mr Hague would like to see a renewed effort from the US to engage the different parties in the peace process and is hopeful that Mr Kerry's visit will herald a new enthusiasm.
Mr Kerry, former Massachusetts senator and 2004 Democratic presidential candidate, has spent almost three decades in the US Senate as a member of the Foreign Relations Committee.