Arguing that the U.S. cannot be "bystanders" as the Syrian government continues to massacre its own people, President Obama Friday afternoon called for the international community to pressure Syrian President Bashar al-Assad to step down.

"All of us who have been seeing the terrible pictures coming out of Syria and Homs recently recognize it is absolutely imperative for the international community to rally and send a clear message to President Assad that it is time for a transition, it is time for that regime to move on and it is time to stop the killing of Syrian citizens by their own government," he said this afternoon after an Oval Office meeting with Danish Prime Minister Helle Thorning-Schmidt.

Secretary of State Hillary Clinton is in Tunis, Tunisia, attending a "Friends of Syria" conference of 60 nations, which called for Assad to end the violence, and to allow humanitarian aid into parts of the country hardest hit by his crackdown. Earlier today, at a press conference, Clinton urged the Syrian military to lay down its arms.

"They're continuing to kill their brothers and sisters is a stain on their honor," said Clinton. "Their refusal to continue this slaughter will make them heroes in the eyes of not only Syrians, but people of conscience everywhere. They can help the guns fall silent."

She also criticized China and Russia for vetoing a United Nations Security Council resolution aimed at Syria.

"It's quite distressing to see two permanent members of the Security Council using their veto while people are being murdered - women, children, brave young men, houses are being destroyed. It is just despicable and I ask whose side are they on?" she asked. "They are clearly not on the side of the Syrian people."

President Obama said he was "encouraged by the international unity that we are developing and the meeting that took place in Tunisia that Secretary Clinton has attended and we are going to continue to keep the pressure up and look for every tool available to prevent the slaughter of innocents in Syria. And this is an area where I think the Prime Minister and I deeply agree that it's important that we not be bystanders during these extraordinary events."

Others in the U.S. have called for further actions to hasten the end of Assad's reign, with Sen. John McCain, R-Ariz., among others calling for the U.S. to help arm the rebels. The Obama administration has been reluctant to go that far, with concerns about possible extremist ties among the insurgent groups. The president said he discussed other issues with Thorning-Schmidt, including counter-terrorism cooperation, putting Afghans in the lead in security in Afghanistan, the European economy, and how "the operations in Libya could not have been as effective" without the Danish armed forces and pilots.

-Jake Tapper, Mary Bruce and Luis Martinez