WASHINGTON — When word circulated in Washington this week that Bouthaina Shaaban, who is a top adviser to President Bashar al-Assad[1] of Syria[2] and who has been under sanction by the United States government, would app ear two blocks from the White House at an antiterrorism news conference, the Obama administration reacted with alarm.

John Kirby, the State Department spokesman, called Dr. Shaaban a "propaganda mouthpiece" of the Assad government, which President Obama[3] has said must go, and the Treasury noted that any transaction with a person designated on one of its blacklists was prohibited.

Dr. Shaaban did not arrive in person on Thursday, but she did appear via Skype video at the National Press Club, where she delivered a 20-minute speech followed by an extraordinary and at times contentious one-hour question-and-answer session with journalists and others.

Seated at a desk with a Syrian flag behind her and one pinned to the lapel of her blazer, Dr. Shaaban defended her government defiantly, assailed the Obama administration as being insufficiently committed to defeating the Islamic State and blamed Western news media for perpetuating what she called a "false narrative" about Syria and Mr. Assad's government.

"We would love American officials to say the truth to their people and to the world about what's happening in our country, because covering the truth is costing us blood," Dr. Shaaban told an audience of several dozen in a nondescript meeting room at the National Press Club, where her image appeared on two large digital screens.

"The question is, why does the United States refuse to cooperate with Russia in fighting terrorism in Syria?"

Dr. Shaaban's presentation was a direct challenge to the approach the Obama administration has taken in Syria, where the United States has worked to empower rebel groups and shepherd international peace talks aimed at bringing about a political transition there and an end to hostilities.

"There's no such thing as moderate opposition" in Syria, Dr. Shaaban said, denouncing the anti-government groups as a "terrorist movement."

She flatly rejected a proposal by the main opposition organization to establish a nationwide truce during the holiday of Ramadan[4].

"The opposition does not want to go to negotiate during Ramadan because they are all Islamists," Dr. Shaaban said.

She deflected many of the criticisms Mr. Obama and top American officials have made of the Syrian government, including accusations by Secretary of State John Kerry that Mr. Assad has a symbiotic financial relationship with the Islamic State, also known as ISIS[5] or ISIL.

"There's no question that the Syrian government never buys oil or sells oil to the terrorists," Dr. Shaaban said.

And she repeatedly sidestepped questions about reports that the Syrian government has blocked humanitarian food and other aid to many thousands of desperate civilians, including in a Damascus suburb, Daraya, that has been blockaded since 2012.

"Daraya is the food basket of Damascus; there's nobody starving in Daraya," Dr. Shaaban said at one point, adding that Syrians are accustomed to eating fresh fruits and vegetables and would weep at the prospect of consuming canned food and macaroni sent by aid agencies. "The Syrian people are able to feed themselves."

She bristled at questions about acts her government or close associates are suspected of committing, including the use of barrel bombs to kill civilians.

"I don't want to give up on the idea of a free press, but you are forcing me to do so, because I'm amazed at how the questions are coming from completely distorted perspective," she said.

The event nearly devolved into chaos after a reporter asked about Dr. Shaaban's association with a former Lebanese information minister, Michel Samaha, who is suspected of being part of a conspiracy to kill Lebanese political and religious leaders.

"I wish you could be constructive," Dr. Shaaban said. "All that you are bringing is something that you think is embarrassing to me, but nothing is embarrassing to me."

"You kill innocent people!" a person who was not identified shouted as the room erupted in jeers and a moderator called for security. The person was not detained.

The gathering was hosted by a nonprofit group calling itself the Global Alliance for Terminating Al Qaeda/ISIS, or Gafta, founded by Ahmad Maki Kubba, an Iraqi-American who is pressing the Obama administration to ally with the Assad government to defeat the Islamic State.

Mr. Kubba compared such a partnership to the cooperation between Franklin D. Roosevelt and Josef Stalin to defeat Hitler.

"We do not have Stalin, but we have worse than Hitler: ISIS and Al Qaeda," he said on Thursday. "If Assad goes, who's going to take over? ISIS and Al Qaeda."

Also attending the event was Bassam al-Hussaini, the Iraqi government's liaison to the Iraqi militias, many of which are backed by Iran.

"These are not Shia militia," Mr. Hussaini said. "These are Iraqis fighting to free the cities of Iraq."

Dr. Shaaban, blacklisted by the Treasury in 2011, said her decision to appear remotely in Washington had nothing to do with the sanctions, but that a visit to the United States in 2005, when one of her aides had been detained for two hours without explanation, had soured her on returning.

"Since that time, I decided that I do not want to visit the United States, although I have received many invitations," she said. "I have an integrity, and I am not ready to be humiliated in any airport."