William Hague: "The challenges of foreign policy are uniquely tricky"
New UK Foreign Secretary William Hague has met his US counterpart Hillary Clinton in Washington, with the war in Afghanistan high on the agenda.
Their talks also included Iran's nuclear programme and the Middle East peace process, Mrs Clinton said.
Mr Hague has said UK troops will be in the country until "their job is done".
But he has stressed the UK wants to see the Afghan government "implementing" commitments it has made on training its own troops and tackling corruption.
The visit is Mr Hague's first overseas trip since the Conservatives and Liberal Democrats formed a coalition government.
US President Barack Obama was the first world leader to call new UK Prime Minister David Cameron to congratulate him after he took power, with the two stressing the importance of the Anglo-US relationship.
At a joint news conference alongside Mr Hague, Mrs Clinton said they had discussed the "shared mission" in Afghanistan and Mr Hague had reaffirmed the UK government's commitment to "achieving long-term stability there".
"The US is deeply appreciative of the British contribution in Afghanistan," she said.
Hillary Clinton said nato-led forces were proving "effective" in Afghanistan
On Iran, Mrs Clinton said Tehran had repeatedly showed its lack of interest in negotiations and was unlikely to respond until there was action from the UN security council.
Mr Hague said the US was "without doubt the most important ally" of the UK and described the two countries' relationship as "an unbreakable alliance".
Earlier, Mr Hague said his visit was an opportunity to "take stock" of the progress being made in Afghanistan, where 9,000 British troops are engaged in fighting the Taliban and training the Afghan army and police.
He has said the government wanted to see more progress on training Afghan troops and police so they could assume security duties, allowing foreign forces to leave but that the UK was not "setting artificial deadlines, arbitrary deadlines, a date of withdrawal".
Mr Hague's talks come days after Afghan President Hamid Karzai visited the US in an effort to repair rocky relations between the two countries.
Robert Fox, of the London Evening Standard, told BBC Radio 4's Today programme that the UK and US governments faced increasing public pressure over the war in Afghanistan.
Clearly there is some public concern that sometimes we simply follow the American tune without having our own independent policy
Professor Malcolm Chalmers Royal United Services Institute for Defence and Security Studies
"There is a very serious Vietnam analogy - on two counts," he said.
"One is, as with Vietnam, the Americans are having to explain to their public backing an evidently corrupt regime, which was the problem in Vietnam as well.
"And also tackling, and this applies to the UK, a mystery - the public is increasingly bewildered about why precisely a main effort has to be made in Afghanistan when they know there is a lot of trouble coming from next door [in Pakistan].
"Is the concept right, and where and how does it end?"
Mr Hague said the UK wanted to see more progress.
After David Cameron was appointed prime minister on Tuesday, Mr Obama was the first world leader to telephone to congratulate him.
The US president told Mr Cameron that America had "no closer friend and ally than the United Kingdom".
Mr Hague has promised a "solid but not slavish relationship" with the US.
"No doubt we will not agree on everything, but they remain, in intelligence matters, in nuclear matters, in international diplomacy, in what we are doing in Afghanistan, the indispensable partner of this country," the foreign secretary said.
Professor Malcolm Chalmers, from the Royal United Services Institute for Defence and Security Studies, said Mr Hague wants to strike a balance "making it clear that this is our most important relationship, but we're not going to be the United States' poodle".
"Clearly there is some public concern that sometimes we simply follow the American tune without having our own independent policy," he said.
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