GWEN IFILL: Next: two takes on the war in Syria.
As the conflict rages on, there are new concerns this week that the Assad government is moving closer to the use of chemical weapons.
Jeffrey Brown has that part of the story.
JEFFREY BROWN: The Syrian civil war has now closed in on President Bashar al-Assad's seat of power, with rebel gunfire ringing out again today in Damascus.
Amateur video also showed new shelling in the capital, as government forces continued a push to retake key suburbs. And while the noise of war grows louder in Damascus, so have fears around the world that Assad may resort to chemical weapons.
The Syrian government has a number of sites containing what is thought to be one of the largest chemical weapons stockpile in the world. It's made up largely of sarin nerve gas, mustard gas and cyanide.
On Monday, President Obama sounded a warning, amid reports of unusual activity at the weapon sites.
PRESIDENT BARACK OBAMA: The world is watching. The use of chemical weapons is and would be totally unacceptable. And if you make the tragic mistake of using these weapons, there will be consequences and you will be held accountable.
JEFFREY BROWN: Assad's regime answered that it would never use such weapons against its own people, but, today, at NATO headquarters in Brussels, Secretary of State Hillary Clinton renewed the warning.
SECRETARY OF STATE HILLARY CLINTON: Our concerns are that an increasingly desperate Assad regime might turn to chemical weapons, or might lose control of them to one of the many groups that are now operating within Syria.
And, so, as a part of the absolute unity that we all have on this issue, we have sent an unmistakable message that this would cross a red line and those responsible would be held to account.
JEFFREY BROWN: That was a view shared by U.N. Secretary-General Ban Ki-Moon speaking at a climate conference in Qatar.
SECRETARY-GENERAL BAN KI-MOON, United Nations: The country has a fundamental responsibility to keep this stockpile of chemical weapons in the safest way. I have warned that if in any case these should be used, then there will be huge consequences, and that they should be accountable.
JEFFREY BROWN: And as fears of chemical warfare grow, the humanitarian crisis has steadily worsened. More refugees streamed into Turkey today, fleeing Syrian air raids.