LONDON >> Efforts to end the international mission in Afghanistan, curtail Iran’s nuclear ambitions and halt the bloody repression of Syria’s opposition will dominate talks opening Tuesday between U.S. President Barack Obama and British Prime Minister David Cameron.

Cameron is heading to the United States for a three-day visit focused on current and looming foreign policy challenges, seeking to pin down details of the planned withdrawal of international forces from Afghanistan by the end of 2014.

The two leaders, who last held talks in person at the United Nations in September, are meeting ahead of a NATO summit in Chicago in May, when a decision on the pace of the exit from Afghanistan is likely to be confirmed.

Their discussions follow the weekend killings of 16 Afghan civilians, allegedly by a lone U.S. soldier, and the deaths of six British troops last week in a roadside bomb blast — the largest loss of life in a single incident for U.K. forces in Afghanistan since 2006.

“As recent days remind us, this remains a difficult mission. We honor the profound sacrifices of our forces and in their name we’ll carry on the mission,” Obama and Cameron wrote Tuesday in a joint op-ed piece published in the Washington Post.

They indicated their talks would focus on exactly when international forces plan to step back from a combat role amid public unease over the still rising death toll in the 11-year-long mission.

NATO had previously agreed that international forces would hand responsibility for the country’s security to local forces by the end of 2014, but U.S. Defense Secretary Leon Panetta has said he now hopes Afghans can assume the lead combat role by mid-2013, meaning the withdrawal of foreign forces could be accelerated.

The miliatary alliance’s plan “includes shifting to a support role in advance of Afghans taking full responsibility for security in 2014,” the leaders wrote in their op-ed.

Last week’s fatalities took Britain’s death toll in Afghanistan since 2001 to 404. Cameron has previously announced that the U.K. will withdraw 500 troops by the end of 2012, reducing the size of the country’s contingent in Afghanistan to 9,000.

“It is worth remembering why we are in Afghanistan,” Cameron told reporters Monday. “We are there to train up the Afghan army and the police so that country is able to look after its own security and make sure that country isn’t a haven for terrorists — without having foreign troops on its soil.”

Setting modest ambitions for the end of the mission, Cameron said Afghanistan must have “at least a chance of stability” by the time foreign forces withdraw.

Alongside formal discussions, Obama plans to fly Cameron to an NCAA Tournament college basketball game in Ohio on Tuesday and will offer the British leader a glitzy White House dinner on Wednesday.

The lavish welcome is in part to offer thanks for the pomp and pageantry during Obama’s visit to Britain last May, when the U.S. president was warmly greeted by Queen Elizabeth II and bonded with Cameron over burgers as the two leaders hosted a barbecue for troops.

It also underscores the strength of ties between Washington and London at a time when some British legislators fear the U.S. is preparing to shift its focus to the Asia-Pacific region.

Parliament’s Joint Committee on National Security Strategy, which includes two former Foreign Secretaries and an ex-spy chief, last week urged Cameron to “reflect deeply on the long term implications” of the U.S. turning its attention away from Europe.

The leaders sought to ally concerns with a gushing statement of friendship. “What makes our relationship special — a unique and essential asset — is that we join hands across so many endeavors. Put simply, we count on each other and the world counts on our alliance,” they wrote in their op-ed.

Obama and Cameron said they plan to discuss how to handle the resumption of negotiations with Iran over its disputed nuclear program. Britain and the U.S. are among the six world powers involved in diplomacy with Tehran.

Both leaders are also seeking to dissuade Israel from launching any pre-emptive strike against Iran, which insists its nuclear program is for peaceful purposes. “We believe that there is time and space to pursue a diplomatic solution,” they wrote.

Last week, Cameron’s Cabinet was briefed by the head of Britain’s MI6 overseas spy agency, John Sawers, on the latest intelligence assessments on Iran’s pursuit of a nuclear weapon.

While Cameron warned last week that “military action against Iran by Israel would not be the right approach,” he acknowledged all options remain possible to handle the issue — including the use of force.

“It is difficult to say that, because no one wants to see conflict in any way, but it is very important that the world sends a message to Iran that a nuclear-armed future is not something that we want to see,” he said.

In Washington, British ambassador to the U.S. Peter Westmacott told reporters there is joint planning to respond to “various scenarios” if Israel does carry out a strike, but he would not give details.

Iran would probably retaliate, and there could be many unpredictable consequences for nations such as the U.S. and Britain that might be seen as supporting Israel. “People are looking at all the options,” he said.

Westmacott said the new negotiations with Iran were welcome, “but we have no illusions. We’ve been here before.”

“If this time they’re serious, we’ll be the first to cheer,” he added.

Cameron and Obama also will consider how to increase pressure on President Bashar Assad, amid the yearlong uprising in Syria during which the U.N. estimates his regime has killed over 7,500 people.

Westmacott said Obama and Cameron had a strong shared conclusion that military action is not the way to solve the crisis.

Cameron has called on nations opposed to Assad to impose economic and diplomatic sanctions and to attempt to document the killings to assist any future prosecutions.

“With our international partners, we’ll continue to tighten the noose around Assad and his cohorts, and we’ll work with the opposition and U.N.-Arab League Envoy Kofi Annan to plan for the transition that will follow Assad’s departure from power,” he and Obama wrote in their joint article.

Cameron’s wife Samantha will join the trip to make a number of appearances alonside first lady Michelle Obama.

The British chief is also being accompanied by Foreign Secretary William Hague and Treasury chief George Osborne. During his visit, Cameron also is expected to meet with New York Mayor Michael Bloomberg and Newark Mayor Cory Booker — but won’t hold talks with any of the Republican presidential candidates.