U.S. Secretary of State Hillary Clinton says the Syrian government's behavior is “reprehensible” and that its actions against the Syrian people are “tragic.”

Speaking to reporters Monday in Prague, Clinton also repeated U.S. warnings against Syria using chemical weapons. She says the issue remains a “red line” that would prompt U.S. action.

Western powers have rejected direct military intervention during the 20-month crisis in Syria, but warned that the use of chemical weapons by the government would trigger a response.

In Syria Monday, opposition activists said government forces shelled Damascus suburbs, a day after carrying out deadly air strikes and rocket attacks against rebel-held areas.

Fighters opposed to President Bashar al-Assad have been trying to secure a perimeter around Damascus from their strongholds on the outskirts of the city.

Meanwhile, the U.S. State Department says it is “hopeful” NATO will approve a plan to deploy Patriot missiles near Turkey's border with Syria. Turkey has asked the alliance for the missiles to bolster its air defenses.

A senior State Department official said Monday that, if NATO signs off on the plan, it will likely still be “a matter of weeks” before the missiles are deployed. NATO foreign ministers are meeting Tuesday and Wednesday in Brussels.

Syrian ally Russia has warned against the missile plan, saying it would not promote stability in the region.

Russian President Vladimir Putin is to meet with Turkish Prime Minister Recep Tayyip Erdogan for talks Monday in Istanbul. The two sides have close trade ties, but remain opposed on their stance toward Syria.

Tensions between Russia and Turkey escalated in October when Turkey forced a Syrian passenger plane traveling from Moscow to land in Turkey because of suspicions it was carrying military equipment. The move outraged Russia, which said the plane was carrying legal radar parts.