U.S. President Barack Obama says the United States is making “very real progress” in Afghanistan, despite new tensions following the shooting deaths of 16 Afghan civilians allegedly by a U.S. soldier.

Mr. Obama spoke at a joint White House news conference with British Prime Minister David Cameron Wednesday.

The president said the “tragic events” of recent days in Afghanistan are a reminder that the mission remains difficult, but that U.S. and NATO forces have made gains against al-Qaida and the Taliban.

He reaffirmed his commitment to the withdrawal timetable that would see the U.S. turn over full responsibility for security to Afghan forces in 2014.

Prime Minister Cameron said the coalition forces in Afghanistan will not build a perfect Afghanistan, but are making tangible progress.

A Reuters/Ipsos poll released Wednesday found U.S. support for the war appears to be waning. Forty percent of Americans told the online poll the recent shooting spree weakened their commitment to the war. Sixty-one percent said the remaining U.S. troops should be brought home immediately.

President Obama describing the violence in Syria on the civilian population as “horrific.” Mr. Obama said the focus remains on getting humanitarian aid into Syria. He said that Syrian President Bashar al-Assad would leave power, saying it was “not a question of if but when.”

The president also said the U.S. remains determined to prevent Iran from acquiring a nuclear weapon. Mr. Obama warned Iran would face more and tougher sanctions if it continued to try to escape or evade its international responsibilities.

Mr. Obama hosted an elaborate arrival ceremony Wednesday morning for the official visit.

He called the relationship between the United States and Britain “one of the greatest alliances the world has ever known” and the “one constant” through the twists and turns of history.

“We stand together and we work together and we bleed together and we build together, in good times and in bad, because when we do, our nations are more secure, our people are more prosperous. The world is a safer and better and more just place. Our alliance is essential. It is indispensable to the security and prosperity that we seek, not only for our own citizens, but for people around the world.”

Mr. Cameron echoed Mr. Obama, calling the U.S.-British relationship “a meeting of kindred spirits.”

“The partnership between our countries, between our peoples, is the most powerful partnership for progress that the world has ever seen. That is why whenever an American president and a British prime minister get together, there is a serious and important agenda to work through. And today is no different. Afghanistan, Iran, the Arab Spring, the need for trade, for growth, for jobs in the world economy, the biggest issues in the world — that is our agenda today.”

Mr. Obama and Mr. Cameron will discuss the upcoming NATO and G-8 summits. Wednesday night, the prime minister will be the guest of honor at a formal White House dinner.

The two leaders wrote in The Washington Post newspaper Tuesday that the U.S.-British alliance is a “partnership of the heart, bound by the history, traditions and values” that the two countries share.

They said they are working with other global economic powerhouses to create jobs, sustain the global recovery, and resolve Europe's debt crisis.

On Tuesday, Mr. Obama shared his passion for basketball with Mr. Cameron, treating him to a U.S. springtime tradition — a college tournament game in Ohio between Western Kentucky University and Mississippi Valley State.

It was the first basketball game Mr. Cameron has attended. He told a television reporter that the game moved fast and the rules were a bit confusing.