The inability to attract high-level defectors has been a continuing source of consternation for Mr. Assad’s opponents and an enduring sign of the government’s resilience, despite its growing international isolation. While the Libyan opposition was able to attract high-level government and military defectors days after the uprising began there — and myriad other officials, from ambassadors to the foreign minister, as the conflict wore on — the Syrian opposition is still waiting for one well-known person to step down and denounce the government.

Even so, a State Department spokeswoman, Victoria Nuland, said the defection of Mr. Hussameldin, while not confirmed, could be significant and that he could be “privy to a lot of information about what Assad has really done to his country.”

“He would be well placed, this particular individual, to understand the impact that the international sanctions that we, that the Europeans, that the Arab League, that other countries are now beginning to put on Syria,” she said.

The video of Mr. Hussameldin, which first surfaced early Thursday in the Middle East, did not specify where or when it had been made, and he could not be reached to verify its authenticity.