The new American Secretary of State, John Kerry, urges the Syrian opposition to attend an international meeting, promising the US is not planning "simply to talk", but is "confident" of results.

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Mr Kerry, who is in London on the first day of an 11-day tour of Europe and the Middle East, said he and Foreign Secretary William Hague "agree that the Syrian people deserve better than the horrific violence that invades their everyday lives".

The secretary of state said he was "very sensitive" to the frustrations over Syria, adding: "I understand the reason people question another meeting, but I'm a new secretary of state. I'm here now beginning a fresh term...and the president of the United States has sent me here."

The Syrian opposition has been invited to attend a Friends of Syria summit in Rome on Thursday; however it is not clear if it will attend. The opposition has said it will only negotiate a peace deal if President Assad steps down, and today refused the Syrian foreign minister's invitation for "dialogue".

Responding to a question from Channel 4 News International Editor Lindsey Hilsum, he said: "I say to the Syrian opposition - we are not coming to Rome simply to talk, we are coming to Rome to discuss the next steps."

US Secretary of State John Kelly visits David Cameron and William Hague in the UK ahead of Syria meeting (Image: Reuters)

Mr Hague, who is due to attend the meeting alongside Mr Kerry, said the UK wants to "significantly" increase the assistance it is giving to opposition forces taking on President Bashar Assad's regime.

"We agree that for as long as a political solution to the conflict is blocked off, the international community has a responsibility to take steps to help prevent the loss of life in Syria, including the terrible loss of life that we have just witnessed in Aleppo.

"That is why in the United Kingdom, we believe we must significantly increase our support for the Syrian opposition, on top of our large contributions to the humanitarian relief effort, and we are preparing to do just that.

I can tell you is that we are determined that the Syrian opposition is not going to be left dangling in the wind wondering if support is coming. John Kerry, US secretary of state

Meanwhile, Mr Kerry told Channel 4 News to "stay tuned" until after the meeting in Rome, when "hopefully we'll have something to announce to you".

After discussions this morning with Mr Hague, the secretary of state said there were "a lot of ideas on the table".

"Some of them I am confident, confident, will come to maturity by the time we meet in Rome," he said.

"What I can tell you is that we are determined that the Syrian opposition is not going to be left dangling in the wind wondering if support is coming", Mr Kerry added.

He said the "moment was ripe" for action, adding: "I would say to Moaz al-Khatib (the leader of the Syrian opposition)...the best way to get results is to join us."

Referring to the 70,000 civilian deaths that have taken place since the Syrian unrest began, Mr Hague said that the UK's policy "can't be static in the face of these events".

Foreign policy will "have to change and develop," he said, adding: "You see the terrible human cost but also the mounting danger of instability in neighbouring countries."

'Window open' for Iran

Ahead of international talks this week with Iran's ruling regime, Mr Hague said he and the US secretary were agreed that Iran's nuclear programme "poses a threat to the peace and security of the world".

Mr Kerry said the possibility of Iran having nuclear weapons is "totally unacceptable". Iran is banned from importing nuclear material under UN sanctions.

He added: "The window for a diplomatic solution simply cannot remain open forever, but it is today...We are prepared to negotiate in good faith and mutual respect.

"The choice really is in the hands of the Iranians and we hope they will make the right choice".

Mr Hague said the Iranians "should not doubt our resolve".

Special relationship

Mr Kerry said it was "no accident" that London was the first stop on his first trip as secretary of state.

The former presidential candidate said: "When you think of everything that binds the US and Great Britain - our common values, our long shared history, our ties of family and friendship - there is a reason why we call this the special relationship, or as President Obama and Prime Minister Cameron wrote, 'a partnership of the heart'.

"In the 20th century our countries fought for freedom side by side and fought for survival together in war, we thrived together in peace and we stood together time and time again in order to meet the world's great challenges.

"In the 21st century, we may face a new and more complex set of challenges, but I absolutely know that we face them together just as we did in the last century.

"And together, it is absolutely clear that our partnership remains stronger than ever."