The United States is stepping up support for the Syrian opposition amid signs events in the 16-month uprising are moving faster on the ground, Secretary of State Hillary Clinton said Tuesday.

After failing to win a U.N. Security Council resolution on Syria after Russia and China imposed their vetoes, the United States is working outside the council to send "a clear message of support for the opposition," she said.

Russia urged its Middle East ally Syria on Tuesday to refrain from using chemical weapons despite its threat to unleash the previously unreported arsenal in case of a foreign attack.

The foreign ministry said it "would like to underline that Syria joined" a Geneva protocol on the non-use of such weapons and "presumes that the Syrian authorities will continue to rigorously abide by its assumed international obligations".

With wealthy Gulf tourists shunning Lebanon this summer, Hani Abdel Malek did not expect that hundreds of Syrians fleeing violence in their homeland would spare his luxury hotel a bad season.

All 82 of the Al-Safat hotel's suites were empty just a week ago, but in just a single day last week, they were all filled up.

More than 10,000 Iraqis have fled Syria to return home since Wednesday fearing for their lives, the U.N. refugee agency said.

"Many of the returnees have expressed their fear regarding the ongoing risks to their safety in Iraq, but said they had little choice, given the security threats in Syria," UNHCR spokeswoman Melissa Fleming said in Geneva on Tuesday.

Renowned Syrian sculptor Wael Qastoun has been tortured to death in the flashpoint central of city of Homs, an artists' association said on Tuesday.

A statement released by the Syrian Coalition of Artists for Freedom said Qastoun "died under torture."

Syrian government troops attacked the Qadam and Assali districts of Damascus on Tuesday, two of the last pockets of rebel-held territory in the city, a watchdog said.

"Syrian regime forces have attacked the Qadam and Assali districts, launching a wave of raids and arrests," the Britain-based Syrian Observatory for Human Rights said.

Syrian rebel forces are stretching the Syria army nationwide, grabbing border posts and countryside regions as they challenge the regime in its symbolic power centers of Damascus and Aleppo, experts say.

An attack in the heart of Damascus last week, claimed by the rebel Free Syrian Army, killed four senior regime members and forced the government to redeploy troops to the capital, leaving gaps elsewhere that the rebels can exploit.

A top Iranian military official warned on Tuesday that Syria's allies "will not allow regime change" and would land "decisive blows" on Damascus's enemies if they entered the growing conflict.

"The Syrian people and the friends of Syria will not allow regime change," Brigadier General Masoud Jazayeri, the deputy chief of Iran's joint armed forces, was quoted as saying on the official website of the Revolutionary Guards, sepahnews.com.

Greece is preparing to close its embassy in Damascus, the foreign ministry said Tuesday, after last week warning about the deteriorating security situation there.

"The foreign ministry is closely observing and constantly evaluating the state of security and has put under way the closure of the embassy of Greece in Damascus," it said in a statement.

Syria has named General Ali Mamlouk as the new head of its national security office and General Rustom Ghazali as the new chief of political security, in a shakeup of the security services after a bombing killed four top regime figures last week, a security source told Agence France Presse on Tuesday.

"General Ali Mamlouk, who was head of state security, is becoming the head of the bureau of national security, with the rank of minister, overseeing the entire security apparatus," the source said.