Post columnist David Ignatius explains the Obama administration’s struggle to confirm if Syria has crossed the "red line” of using chemical weapons. (The Fold/The Washington Post)

U.N. Secretary General Ban Ki-moon renewed an appeal Monday to Syria to allow U.N. chemical weapons experts into the country, saying that on-site inspections are essential to “establish the facts and clear up all the doubts” surrounding the reported use of the banned weapons in Syria’s escalating civil war.

Ban’s remarks — delivered the with chief U.N. chemical weapons inspector, Ake Sellstrom of Sweden, at his side — followed allegations by several countries, including the United States, that chemical weapons were likely used in Syria in recent months.

Ban spoke hours after Syrian Prime Minister Wael al-Halki narrowly escaped a car bomb that targeted his convoy Monday morning as it moved through an upper-class neighborhood of Damascus. State television said he was unharmed. At least nine people were killed and 17 injured in the blast, according to Mayadeen television, a network loyal to Assad.

The bombing in the Mezzeh area of Damascus, close to government ministries and several embassies, underscored the steadily expanding reach of Syria’s rebel forces, even as the government of President Bashar al-Assad has sought to halt a gradual rebel encroachment in the countryside surrounding the capital in recent weeks.

Syria invited the United Nations last month to conduct an investigation into its claims that Syrian rebels attacked Syrian government forces with chemical weapons on March 19 near Aleppo. Syrian officials said 26 people died in the attack, including some government troops.

But Syria balked after Britain and France urged the United Nations to also investigate Syrian opposition claims that the government employed chemical weapons in three Syrian cities: Aleppo, Damascus and Homs.

Last week, the Obama administration weighed in, telling Congress that the U.S. intelligence community believes chemical weapons, notably the nerve agent sarin, may have been used by Syrian government forces “on a small scale.” But the White House said it could not confirm “how the exposure occurred and under what conditions.”

President Obama said the administration is still looking for conclusive evidence, and he pointed to the U.N. inspection efforts as the possible key. Britain, France and Israel have also said there is evidence that the Syrian government likely used the banned weapons against rebel fighters.

Speaking before a meeting with Sellstrom on the status of the U.N. probe, Ban said, “I take seriously the recent intelligence report of the United States about the use of chemical weapons in Syria.” He urged the Syrian government “to allow the investigation to proceed without delay and without any conditions.”

Ban said that “a credible and comprehensive inquiry requires full access to the sites where chemical weapons are alleged to have been used.” An advance team of U.N. inspectors is positioned in Cyprus to deploy to Syria within 24 to 48 hours, he said.

Sellstrom was in Britain last week to examine evidence that British authorities say indicates the use of a nerve agent at Syrian sites, and Ban said last week that the United Nations has already been in contact with the United States to discuss its claims.

Syrian government officials pointed to Monday’s car bombing in Damascus as evidence that Syria’s opposition forces were unwilling to participate in a “political solution” to the country’s crisis, the state-run Syrian Arab News Agency (SANA) reported.

Interactive Grid: Keeping track of the conflict in Syria through videos, images and tweets.

Halki, appearing unscathed, spoke to Syrian state television after the attack, vowing to continue “fighting terrorism and terrorists.”

“These explosions always come after accomplishments are made by the Syrian Arab Army, and after accomplishments made by the government,” he said at a news conference aired by Syria’s state-run al-Akhbariya network.

Halki went on to attend several afternoon cabinet meetings as scheduled, state media reported.

Opposition activists claimed that more than 50 other people were killed Monday in shelling and clashes across the country.

The Syrian Observatory for Human Rights, a Britain-based watchdog group, said in a statement shortly after the bombing that at least one of the prime minister’s bodyguards was killed and another critically injured. The group relies on a network of activists throughout the country for its reporting.

Television footage showed firefighters shooting water onto the wreckage of a twisted and smoldering car, amid a landscape of scattered debris on a Damascus street. At least one bus also appeared to be battered and burned.

One resident of the Mezzeh neighborhood said she ran outside in her nightclothes after hearing the blast, frantic to find her son who attends school nearby. “I saw the black column of smoking gathering in the distance near my son’s school, and my heart fell,” the woman said. Like many Syrian activists and witnesses of the nation’s spiraling violence, she declined to be identified for fear of reprisal from the government or other groups.

The woman, interviewed by telephone, said she found a scene of chaos when she reached the site of the blast. Dozens of police ran amid the debris, where she said several cars were on fire. The sounds of sirens from approaching ambulances and the smell of burning rubber filled the air. Spectators, including several parents and children from the nearby school, stood watching the scene in tears, she said.

Multiple television news outlets allied with the government said the bombing was a “suicide” attack.

No group immediately claimed responsibility for the attack, which bore the hallmark of Sunni Muslim extremist groups in Syria, but SANA blamed “terrorists.”

The government has repeatedly cast its rivals — ranging from civilian opposition activists to hardened extremist fighters — as “terrorists” in a bid to win support for its battle, both domestically and abroad.

A bombing that targeted the national security headquarters in Damascus during a cabinet meeting in July 2012 killed Syria’s defense minister and his deputy, Assad’s brother-in-law, and wounded the interior minister. In December, a car bomb and other explosions damaged the Interior Ministry and caused dozens of casualties.

Syria’s civil war has grown increasingly sectarian in more than two years of bitter bloodletting between government forces, dominated by Assad’s minority Alawite sect, and a predominantly Sunni rebel movement.

In recent weeks, violent clashes have raged in areas of the countryside surrounding Damascus, where government forces have shelled towns and neighborhoods in an attempt to rout Syrian rebels who have been making slow gains on the capital.

The Syrian Observatory for Human Rights said at least 100 people were killed in a five-day government offensive in the town of Jdeidet al-Fadel this month. Other activists said the death toll was likely in the hundreds.

The increasingly complex nature of Syria’s war, which has evolved from a peaceful protest movement repressed by the government to a divided battleground of militarized forces backed by sectarian groups and regional powers, has presented Washington and its allies with a dilemma in seeking a solution — or possible intervention.

The Assad government, meanwhile, has used bombings such as Monday’s attack to support its argument that its collapse would yield only a state dominated by terrorists hostile to Western interests.

Hauslohner reported from Beirut. Ahmed Ramadan in Beirut contributed to this report.