Business

WASHINGTON — As the Syrian opposition neared a decision on whether to attend an international peace conference next week, Secretary of State John Kerry offered a public assurance on Thursday that the Obama administration had not pulled back from its goal of establishing a transitional government that would not include President Bashar al-Assad.

In a quickly arranged appearance before reporters, Mr. Kerry criticized “revisionism” and attempts to “muddy the waters” over the reasons the peace conference has been organized.

“Any names put forward for leadership of Syria’s transition” at the conference, Mr. Kerry said, “must be agreed to by both the opposition and the regime.”

“This means that any figure that is deemed unacceptable by either side — whether President Assad or a member of the opposition — cannot be a part of the future,” Mr. Kerry said.

Mr. Kerry did not take any questions. But State Department officials said his aim was to reassure moderates among the Syrian opposition, which is expected to decide as early as Friday whether to attend the peace conference.

The conference is scheduled to begin on Wednesday in Switzerland.

The moderate Syrian opposition has long been concerned that its already limited influence among rebel fighters may be further diminished if it is drawn into open-ended talks with a government that does not intend to hand over power.

Those worries have been heightened by a recent letter from Walid al-Moallem, the foreign minister of Syria, to the United Nations’ secretary general, Ban Ki-moon, that appeared to challenge the conference’s purpose.

Mr. Moallem, who is to lead the Assad government’s delegation, wrote that “certain points” in the United Nations’ official invitation were “in conflict with the legal and political position of the State of Syria” and did not “meet the supreme interests of the Syrian people.”

The letter said nothing about arranging for a transitional administration to govern Syria, and it suggested that the Assad government’s main concern was to “fight terrorism.”

The letter has not been officially released by the United Nations, but diplomats have confirmed the authenticity of a translated copy that has been circulated by the Syrian opposition.

Oubab Khalil, the chief of staff of the opposition’s Washington office, said the letter shows that the Assad government does not accept that the main point of the conference is to negotiate a transitional government.

Jen Psaki, the State Department spokeswoman, told reporters that Mr. Moallem’s letter was an example of the “revisionism” that Mr. Kerry had criticized.

But the State Department also appeared to be at pains to dispel rumors within the ranks of the opposition that the Obama administration might be tempted to cut a deal with Mr. Assad so it could better contain the militants affiliated with Al Qaeda who have joined the civil war in Syria.

Mr. Kerry said on Thursday that it was the Assad government that had turned Syria into a “magnet for jihadists and extremists” by brutally cracking down on its opponents.

“And so on the eve of the Syrian opposition coalition general assembly meeting” on whether to attend the conference, Mr. Kerry added, the United States “urges a positive vote.”

As the United States sought to rescue the conference, Russia and Iran, who have each given military and political support to the Assad government, were also strategizing.

While Russia agrees that the purpose of the conference is to establish a transitional government by “mutual consent,” it has not acknowledged that Mr. Assad has to give up his post.

Iran has not formally accepted the terms of the conference, and has not been invited to attend, but it has been consulting with Moscow and the Assad government.

In an unusual three-way meeting in Moscow on Thursday, Russia’s foreign minister, Sergey V. Lavrov, talked with Mr. Moallem and the Iranian foreign minister, Mohammad Javad Zarif.

The gathering spurred speculation that the three nations were devising a common strategy that would enable Mr. Assad to maintain his hold on power.

“We have nothing to hide,” Mr. Lavrov said in a joint news conference with Mr. Zarif. “We have no hidden agenda.”