The Obama administration says it is still considering a no-fly zone for northern Syria as cross-border shelling with Turkey threatens civilians on both sides.
With more Syrian tanks along the border, artillery fire between Syria and Turkey risks spreading violence, said
U.S. State Department spokeswoman Victoria Nuland.
"The Turks have been very consistent that they are striking back strongly and proportionally every time they take an attack across the border," Nuland said. "This is extremely dangerous and goes to the point that we’ve been making about the danger of this conflict spilling beyond borders."
Turkey's response is about more than border security, said U.S. Institute of Peace analyst Steve Heydemann.
"It's also very clearly an effort on the part of the government in Ankara to change the political equation and to bring much greater pressure to bear not only on the Assad regime itself, but on the international community about what is at stake in this conflict and why the international community needs to do much more than it has," Heydemann said.
With more than 100,000 Syrian refugees in Turkey, Washington is weighing the no-fly zone for areas of northern Syria controlled by Assad opponents.
"We continue to talk to partners about how, what, why, exactly the elements that might go into some of these things that people have proposed, including a no-fly zone, but we haven’t made any decisions at this stage," Nuland said.
President Assad realizes a no-fly zone would give his rebel opponents an "extraordinary advantage, said Heydemann.
"The Syrians view this escalation of conflict across the Syrian-Turkish border as another effort to establish Syria's determination not to be intimidated by the superior military of Turkey, not to be intimidated by the threat that NATO might support Turkey."
He said Assad's government is feeling emboldened by outside help at a time when international military support for the rebels is limited.
"They feel as if Russia, China, Iran, Hezbollah are very firmly on their side," Heydemann said. "They sense the prevarication of the international community in increasing its support for the opposition. And they feel that gives them the advantage."
Lebanon's Hezbollah faction says it does not have fighters in Syria. Still, the United States is concerned about growing Hezbollah and Iranian influence in Syria and is sharing those concerns with Russia, Nuland said.
"If Moscow is concerned about these kinds of things -- they have expressed concern about what could come after Assad -- and our point is, what is coming now with Assad still in power, increasing efforts by extremists of all kinds and by Iran to make trouble that could spread even beyond borders," she said.
Nuland added the United States is working with allies who supply weapons to Syrian rebels to ensure that those arms are not going to groups that are being infiltrated by extremists.