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 key gains.

“What's undeniable though, and what we can never forget, is that our forces are making very real progress dismantling al-Qaida, breaking the Taliban's momentum and training Afghan forces so that they can take the lead and our troops can come home.”

Mr. Obama also said the timeframe for withdrawing U.S. troops remains.

“In terms of pace, I don't anticipate at this stage that we're going to be making any sudden, additional changes to the plan that we currently have.”

That plan would see U.S. and NATO turn over full responsibility for security to Afghan forces in 2014.

Prime Minister Cameron said he also remains optimistic, and that Afghanistan is less of a threat now than before.

“If you compare where we are today with where we've been two, three years ago, the situation is considerably improved.”

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.

Another key concern for the two allies is the ongoing violent and deadly government crackdown in Syria.

President Obama called the violence “horrific” and warned time is running out for Syrian President Bashar al-Assad.

“Just as the regime and security forces continue to suffer defections, the opposition is growing stronger. I'll say it again. Assad will leave power. It is not a question of if, but when.”

The British prime minister called the violence “simply appalling” and warned those responsible would be held to account.

“No matter how long it takes, people should always remember that international law has a long reach and a long memory. And the people who are leading Syria at the moment and are committing these crimes need to know that.”

The president and prime minister also took aim at Iran, saying time is running out for a peaceful resolution to the nuclear issue.

“Because the international community has applied so many sanctions, because we have employed so many of the options that are available to us to persuade Iran to take a different course that the window for solving this issue diplomatically is shrinking.”

The president said U.S. strategy is aimed at preventing Iran from acquiring a nuclear weapon, and not on containing a nuclear Iran.

Mr. Obama hosted an elaborate arrival ceremony Wednesday morning for the official visit, which included a luncheon at the State Department. A formal dinner at the White House is scheduled for the evening.

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.

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NEW SOUNDBITES:

OLD SOUNDBITES:

President Obama: “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.”

Prime Minister Cameron: “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.”