John Kerry, the US secretary of state, has said Syrian President Bashar al-Assad will have to step down as part of any political solution in Syria.
Kerry made the comments on Thursday in Rome while meeting his Jordanian counterpart, Nasser Judeh, during the third day of talks on the two-year conflict.
He said all sides were working to "effect a transition government by mutual consent of both sides, which clearly means that in our judgement President Assad will not be a component of that transitional government".
Kerry also officially unveiled $100m in additional US humanitarian aid for Syrian refugees, almost half of which will go to help Jordan struggling to cope with a tide of people fleeing the war.
About 2,000 people are flooding across the border into Jordan every day, and the country now hosts nearly 525,000 refugees, Judeh said at the start of the talks in Rome.
"We have 10 percent of our population today, in the form of Syrian refugees. It is expected to rise to about 20 to 25 percent given the current rates by the end of this year, and possibly to about 40 percent by the middle of 2014," he said.
"No country can cope with the numbers as huge as the numbers I've just described," he said, adding Jordan was very grateful for the help of the international community.
Plans for an international conference to try to find a solution to the crisis were also continuing, Kerry said, after he agreed in talks in Moscow that he and Sergei Lavrov, the Russian foreign minister, would work in tandem on the issue.
He had spoken with most of the foreign ministers from the countries involved and there is a "very positive response and a very strong desire to move to this conference and to try to find, at least exhaust the possibilities of finding, a political way forward".
Ban Ki-moon, the UN chief, had also been in touch, so "we are going to forge ahead very, very directly to work with all of the parties to bring that conference together," Kerry added.
No venue has yet been identified, but Geneva could host the talks that are likely to be held by the end of May.
The US secretary of state said the US ambassador to Syria, Robert Ford, also met the Syrian opposition in the Turkish city of Istanbul on Wednesday to discuss the way forward.
Since the war erupted to oust Assad, more than 1.5 million Syrians have fled the country into neighbouring countries, including Turkey, Jordan and Lebanon, vastly straining their resources.
Up to four million more could be displaced within the country as they seek to flee the fierce fighting, which has already claimed more than 70,000 lives, according to the UN.