The Syrian government criticized the U.S. actions, saying Washington is unsuitable to act as a broker at any peace negotiations.
"Washington's decision to send arms to terrorists in Syria confirms that the American administration is not objective in efforts to find a political solution and hold an international conference in Geneva," Syrian state TV said, citing an unidentified Foreign Ministry official. Assad's government routinely refers to opposition fighters as "terrorists."
"The American intentions seek to continue the cycle of violence and terrorism in Syria in order to destabilize ... the region," the statement said.
The Syrian government has gone on the offensive and has succeeded, with the help of Lebanese Hezbollah militants, in pushing back rebels near Damascus and in central Homs province.
The White House acknowledged that momentum has shifted as Hezbollah and Iran have helped Assad's forces.
The rebels are lashing out with stepped up mortar attacks on Damascus, the seat of Assad's power, and with car bombs that target regime strongholds and security installations.
Thursday's car bomb exploded in Jaramana, a suburb just few kilometers (miles) southeast of Damascus that is overwhelmingly pro-regime.
The state news agency SANA reported that the blast caused heavy damage to nearby buildings and destroyed many cars. TV footage showed mangled cars and heavily damaged residential buildings. No group immediately claimed responsibility for the bombing.
The Britain-based Syrian Observatory for Human Rights initially put the death toll at 10 but later raised it to 17, saying several of the wounded had died.
Also Thursday, the Syrian National Coalition cast doubt on a mission by U.N. experts to investigate the alleged use of chemical weapons in the civil war.
Both sides accuse each other of using the banned weapons. Swedish chemical weapons expert Ake Sellstrom and U.N. disarmament chief Angela Kane met with Syrian officials Tuesday in Damascus to try to hammer out terms for a possible probe.
The coalition said it was worried the experts would be swayed by Assad, whom they predicted would try to restrict their movements.
Ban said the U.N. team discussed with Syrian officials "the modalities of investigation."
"Since they're coming out of Syria now, we'll get the report soon," he said.
In Lebanon, a senior Hezbollah official warned European countries that there will be "repercussions" to their decision this week to place the group's military wing on the bloc's list of terrorist organizations.
Hezbollah's participation in the Syrian war alongside Assad's forces was among the reasons for the EU decision earlier this week.
Ammar al-Moussawi, head of Hezbollah's foreign relations department, spoke after meeting with Angelina Eichhorst, the EU ambassador to Lebanon. He suggested after the talks that she was not given a warm reception.
"No one can condemn me with one hand, then extend the other to shake hands," he told reporters in Beirut after the meeting.
Lederer reported from the United Nations. Associated Press writer Zeina Karam contributed to this report from Beirut.