U.N.-Arab League envoy Kofi Annan is weighing an invitation to meet with President Bashar al-Assad in Syria, a senior U.N. source said Thursday, the same day that a deadly blast in Damascus drew widespread condemnation and finger-pointing in the volatile nation.

The invitation was extended via Syrian Foreign Minister Walid Moallem over the past few days, and thus wasn't necessarily tied to Thursday's attack.

The former U.N. secretary-general brokered a cease-fire deal, to which key parties had ostensibly agreed, that was to take effect April 12. Since then there have been some calmer days but violence has continued: The Local Coordination Committees of Syria, a network of opposition activists, reports at least 1,025 people have been killed -- some of them executed and tortured to death -- since that date.

Syria has been mired in unrest since March 2011, when al-Assad's forces began cracking down on anti-government demonstrators. The United Nations has said more than 9,000 people have been killed nationwide since then, while opposition groups put the toll at more than 11,000.

Even amidst all that bloodshed, Thursday was a milestone day.

Syrian authorities two "booby-trapped cars" filled with more than a ton of explosives blew up at an intersection in densely populated Damascus neighborhood of Qazzaz as people headed out to start their days.

State television showed panicked residents running down bloody streets strewn with body parts and burned cars. The government said the blasts led to scores of car crashes.

"The place looks like hell," a man told Syria state TV, describing "burned corpses all over the place" and people dead in their homes.

The opposition Syrian Observatory for Human Rights said at least 59 died and the nation's intelligence agency building was destroyed in what it called the single deadliest attack since Syrian forces began cracking down on dissenters in March 2011.

Syria's U.N. ambassador Bashar Jaafari said 56 civilians and security forces died and 372 suffered "grave injuries in what he called a "cowardly" attack in the vicinity of four schools.

Al-Assad's government faults "terrorists," the term it uses to describe the opposition and rationalize security forces' crackdown.

But two groups part of that effort -- the Syrian National Council and the National Coordination Committee for Democratic Change -- blamed the regime. Opposition members have accused al-Assad's forces of high-profile bombings in the large cities of Aleppo and Damascus in recent months to undermine the resistance's credibility.

"This is a government-planned attack," said Ausama Monajed, the adviser to the Council's president. "We are in touch with the armed resistance."

Brig. Gen. Moustafa el-Sheikh, head of the rebel Free Syrian Army's military council, said that "no other parties in Syria ... are technically capable of making such a huge explosion, except for the regime itself."

"Not even al Qaeda can do that," he said.

Analysts said the attack raises concerns about the presence of jihadist elements in Syria, noting the Damascus strike resembles suicide car bombings during the sectarian civil warfare prevalent in the last decade in Iraq.

Bill Roggio, an analyst on terror and military issues, said he believes the attack "very likely" was carried out by an al Qaeda-linked militant group called the Al Nusrah Front for the People of the Levant, which has claimed credit for recent suicide attacks in Damascus and Aleppo.

He said another jihadi group called the Al Baraa Ibn Malik Martyrdom Brigade also has surfaced, and that al Qaeda in Iraq has had a "strong presence" in Syria. Foreign fighters entered Iraq through Syria during the war there.

Jeffrey White, a defense fellow and analyst at the Washington Institute for Near East Policy, concurred that Thursday's suicide strike is "coordinated (and) very destructive," much like what al Qaeda elements in Iraq claimed credit for.

While Roggio, the managing editor of the Long War Journal blog, called it "very concerning" that opposition groups have "ignored" or denied the activities of terrorist groups, White said opposition members aren't necessarily ignoring an uptick in terrorist activity.