Syrian gunmen in the Bab al-Tabbaneh neighborhood during clashes with Alawites, an offshoot of Shiite Islam to which Syrian President Bashar al-Assad belongs, in the Jabal Mohsen neighborhood in the northern Lebanese port city of Tripoli. (Joseph Eid/AFP/Getty Images)

The brother-in-law of Syrian President Bashir al-Assad, who also is the troubled nation's former intelligence chief, is dead after an apparent assassination attack Sunday in Damascus, Syrian opposition leaders tell U.S. News & World Report.

The Free Syrian Army, an armed opposition group that has been fighting Assad's military for months, said Sunday it had killed six senior members of Assad's cabinet. The Assad regime denied those claims, according to official state media reports, but it is often difficult to get accurate information out of the country.

It remained unclear into Monday evening in Damascus whether Assad's brother-in-law, Assef Shawkat, had been killed in the alleged attack.

But a senior member of one Syrian opposition group says the organization has learned Shawkat has died.

The source says that "a special unit from the Free Syrian Army poisoned the meal" being consumed Saturday evening by Shawkat and other senior Assad regime officials. The source said opposition leaders do not believe the special unit received assistance from U.S., Western or regional intelligence agencies or militaries.

A White House spokesman said he could not confirm the claim of Shawkat's alleged death.

Media reports from the region stated Assad's interior minister, defense minister and a deputy to his vice president also had been attacked.

The hospital where the officials allegedly were taken "has been locked down for two days," the opposition official says.

If Shawkat and other senior Assad cabinet officials are dead, "it would be a huge setback for the regime," the opposition official says, because "this was the group leading the [conflict] for the regime."

The opposition official's claims that Shawkat has died in the Free Syrian Army's multiple-assassination plot comes hours after Ivo Daalder, Washington's ambassador to NATO, told reporters on Sunday evening that there is "no planning going on that's related to a NATO role in Syria."

Several Middle East nations have quietly been working to get more sophisticated arms to Syrian rebels, according to sources and media reports. The multiple-assassination plot suggests rebel forces are growing in sophistication and capability.

John T. Bennett covers national security and foreign policy for U.S. News & World Report via the DOTMIL blog. You can contact him at jbennett@usnews.com or follow him on Twitter.