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The blast blew off the front of the first vehicle and sent up a plume of smoke as people screamed and frantically ran for cover. The four cars are then seen slowly driving away.

It was not clear how close the observers were to the funeral shootings, but if confirmed, a regime attack on civilians directly in front of the observer mission could put pressure on them to describe publicly what they are seeing in Syria. They report back to the United Nations but have not publicized their findings.

Also Wednesday, a Turkish official said the situation in Syria and discussions on the possibility of a NATO intervention were bound to come up during a NATO summit in Chicago next week. So far, the international community has shown little appetite for getting involved in another Arab nation in turmoil.

The official, who spoke on condition of anonymity in line with Turkish government regulations, said NATO could become involved if the U.N. Security Council approved an intervention — a move considered unlikely given Russia and China’s support of Mr. Assad — or if any of the NATO members feels threatened and calls for protection from the alliance.

The official said Turkey would call for NATO protection if “our national security and national interests are threatened or if there is an attack from Syria,” though he added “there is no such situation at present.”

Syria‘s state-run TV, meanwhile, reported Wednesday that authorities had released 250 people who were involved in the uprising. Mr. Assad has issued several pardons releasing thousands of detainees since the crisis began.

The Observatory also said Syrian forces opened fire at the Naziheen Palestinian refugee camp in the southern city of Daraa, killing four people. The pro-government TV station Ikhbariyah blamed members of “an armed terrorist group,” saying they fired two rocket-propelled grenades at the camp, killing a 4-year-old girl and wounding 15 other people.

Activists also reported intense shelling by Syrian troops of the rebel-held central town of Rastan, which witnessed intense clashes between troops and rebels earlier this week.

In the central city of Homs, activists said Syrian troops stormed the Shammas neighborhood late Tuesday, killing at least 15 people. The Observatory said some of those killed were subjected to “summary executions.” An amateur video posted online showed about 10 dead men lying on the floor of a room said to be in Shammas neighborhood.

The reports could not be independently confirmed.

The Syrian uprising began with mostly peaceful protests calling for change, but a relentless government crackdown has led many in the opposition to take up arms. Some soldiers also have switched sides and joined forces with the rebels.

The United Nations estimates the conflict has killed more than 9,000 people.

Tensions have spilled over the border, with Lebanese tribesmen loyal to Mr. Assad kidnapping Syrian opposition supporters to exchange them with relatives abducted in Syria recently.

Members of Lebanon’s Jaafar tribe kidnapped seven anti-government Syrian citizens recently and exchanged them Wednesday for two men kidnapped in the Syrian border town of Zeita, according to Lebanese security officials, who spoke on condition of anonymity in line with regulations.

Associated Press writer Suzan Fraser contributed to this report from Ankara, Turkey.