U.S. Secretary of State Hillary Clinton was in Istanbul on Saturday for talks with Turkish leaders on how to support Syrian rebels and prepare the transition to a post-Bashar Assad era.
Clinton planned to meet President Abdullah Gul as well as the country's prime minister and foreign minister.
She was also expected to meet with Syrian opposition activists for the first time.
U.S. State Department officials said the talks would focus on coordinating U.S. and Turkish support for the rebels. The United States is already providing non-lethal assistance, including communications equipment and medical aid. Additional financial help is expected to be announced by Clinton at a press conference scheduled for 1030 GMT.
The visit comes amid a series of high-level Syrian defections and a surge in violence in several areas of Syria, including the northern city of Aleppo, the country's economic hub.
U.S. officials have declared for weeks that the end is in sight for Assad, and have shifted their focus beyond the failed UN peace initiative to the question of what comes afterwards.
Yet there is uncertainty over how closely the Syrian opposition groups are working together, and whether some groups are acting as proxies for countries in the region or even terrorist groups.
U.S. President Barack Obama's administration is under increasing pressure to get more involved in the 17-month conflict.
More U.S. sanctions targeting Syria are planned in the coming days.
Turkey is seen by many analysts as a key element. At least 50,000 Syrian refuges have sought protection there, and trade and culture historically have linked the two countries
In Syria itself, government forces backed by jet fighters, heavy artillery and tanks pounded areas held by rebels in Aleppo, where the sides have been fighting for three weeks now, said activists.
"We are fighting on. But the size of the regime forces around the city is giving us a hard time in getting fresh supplies of ammunition into Aleppo," Abu Omar al-Halabi, a rebel commander, told dpa by phone from the embattled city.
The battle for Aleppo could decide the course of the conflict in
Syria. However, the fighting inside the country's biggest city seems to have settled for a stalemate.
"In some areas in Aleppo, we have inflicted heavy casualties on
the regime forces. In other areas and because of their heavy weapons, they have won," al-Halabi noted.
Elsewhere, the opposition said heavy gunfire could be heard around aPalestinian refugee camp in the capital Damascus.
The fighting between Assad's forces and rebels around the
Yarmuk camp has forced many Palestinian refugees to flee to neighboring Lebanon, said activists.
Meanwhile, Syrian troops were shelling the dissident town of al-Tal, on the northern outskirts of Damascus, said the opposition Syrian Observatory for Human Rights.
The violence has prompted an exodus of local residents who fear that a major military offensive is in the making, added the London-based organization.
The total number of people who have fled Syria to neighboring countries rose to at least 147,000 last week, according to the UN refugee agency.
Government troops have stepped up their raids on pro-rebel districts inside and around Damascus since July 18, when a bomb attack killed four key members of Assad's inner circle.
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