The United Nations under attack in Syria.

Amateur video out of Homs, which could not be independently verified by Reuters, appears to show men attacking a U.N. vehicle.

The video, which could not be independently verified by Reuters, comes as U.S. Secretary of State Hillary Clinton is warning Russia that it is in its interest to play a constructive role in Syria.

(SOUNDBITE) (English) U.S. SECRETARY OF STATE HILLARY CLINTON, SAYING:

"Russia says it wants peace and stability restored. It says it has no particular love lost for (Syrian President Bashar al-) Assad and it also claims to have vital interests in the region and relationships that it wants to continue to keep. They put all of that at risk if they do not move more constructively right now."

On Wednesday Russia's foreign minister defended his country's sale of arms to Syria, while accusing the U.S. of supplying rebels with weapons - a charge the United States denies.

Russia is one of Syria's principal defenders and has stymied efforts by Western powers to pressure President Bashar al-Assad into stepping down.

Deborah Lutterbeck, Reuters