President Barack Obama has said the prospect of international military intervention in Syria is premature and could lead to a civil war.
Speaking at a White House news conference on Wednesday, Obama said military intervention could lead to more deaths in Syria.
Obama says he and David Cameron, the British prime minister, discussed possible "immediate steps'' their countries could take in order to make sure humanitarian aid is being provided to the Syrian people.
The United Nations has estimated that more than 7,500 people have been killed in a year-long struggle between President Bashar al-Assad's regime and opposition forces.
The two leaders also outlined plans to shift the NATO war effort in Afghanistan towards a back-seat advisory role while Afghan forces increasingly take the lead, but stressed that the two nations remain committed to the mission there.
Obama gave his fullest endorsement yet for the mission shift, but he said the overall plan to gradually withdraw forces and hand over security in Afghanistan will stand.
Obama said he anticipated no "sudden, immediate changes to the plan we already have", for bringing forces home.
The US and Britain have the largest fighting forces in Afghanistan, where the combat is in its 11th year. The US, Britain and other NATO nations have already agreed to keep forces in the country through 2014, when Hamid Karzai, the Afghan president, will leave office.
"At the upcoming NATO summit in my hometown of Chicago, we'll determine the next phase of transition,'' Obama said after a private meeting at the White House with visiting David Cameron, the UK prime minister.
"This includes shifting to a support role next year in 2013 in advance of Afghans taking full responsibility for security in 2014. We're going to complete this mission and we're going to do it responsibly.''
Obama acknowledged the drop in public support at home for the war.
"People get weary", after long wars, the president said, but he also said he thought most people in both the US and Britain understood the reasons for continuing the fight.
Cameron, who joined Obama for a joint Rose Garden news conference, said security was better in Afghanistan and he praised the US strategy to add more than 30,000 forces in a "surge'' against the Taliban-led militants in 2009.
"The situation is considerably improved,'' Cameron said, and the goal of keeping Afghanistan from again becoming a terrorist haven is achievable by the end of 2014.
Following the summer fighting season, Obama said NATO allies would look at how to continue drawing down forces at a gradual pace.
The Obama-Cameron meeting came in advance of May's NATO summit in Chicago, where a decision on the timetable for withdrawal from Afghanistan is expected to be confirmed.
The White House discussions follow the weekend killings of 16 Afghan civilians, allegedly by a lone US 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 British forces in Afghanistan since 2006.
On Iran, Obama insisted there is still "time and space'' for a diplomatic solution, in lieu of a military strike to set back Iran's progress towards a possible bomb, but said "the window for diplomacy is shrinking".
"We are determined to prevent Iran from acquiring a nuclear weapon,'' Obama said, adding that he had sent a message "personally'' to the Iranian leadership that it should re-enter international arms talks in good faith.
"Tehran must understand that it cannot escape or evade the choice before it. Meet your international obligations or face the consequences.''
The White House lavished Cameron with all the pomp and pageantry of a state visit as the two allies stressed their unity in dealing with hot spots such as Iran, Syria and Afghanistan.