Even as planning intensifies for a Geneva peace conference on the war in Syria, the emergence of a group affiliated with Al Qaeda has undermined the chances of negotiating an end to the conflict, a senior State Department official said on Monday.
By challenging moderate Syrian rebels, the group, the Islamic State of Iraq and Syria, has forced them to fight on two fronts and divert resources from their battle with the government of President Bashar al-Assad, the official said.
And by presenting an extremist face to the world, the official said, the group is aiding Mr. Assad’s efforts to portray the conflict in Syria as a tug of war between the government and jihadists.
“That has to give the regime comfort and confidence, and it will make the task of extracting concessions from the regime at the negotiating table more difficult,” said the official, who declined to be identified in keeping with the State Department’s protocol for briefing reporters on active diplomacy.
Secretary of State John Kerry, who arrived in Paris on Monday for the first of three days of Middle East diplomacy talks in European capitals, was scheduled to meet with diplomats from 10 nations in London on Tuesday to discuss preparations for a Syria peace conference.
A principal goal of the peace conference, which is expected to be held next month in Geneva, although no date has been set, is the establishment of a transitional government “by mutual consent” of Syrians that would not include Mr. Assad.
But the senior State Department official said fighters from the Islamic State of Iraq and Syria, known by its initials, ISIS, had hampered the flow of American and other foreign assistance to the moderate resistance inside Syria, diluting the effort to increase the leverage on the Syrian leader.
“It has been very disruptive to our cross-border efforts — very disruptive,” the official said.
As if to illustrate Mr. Assad’s determination to ignore the demands of the United States and other nations that he yield power, the Syrian president suggested in an interview with a Beirut television station that he could seek re-election next year.
“Personally, I don’t see any obstacle to being nominated to run in the next presidential elections,” he said in the interview with the television station, Al Mayadeen.
Mr. Assad said that it was too soon to decide but that his choice would be based on “the will of the people.”
Mr. Assad appeared to receive a lift on Monday when a prominent rebel leader in southern Syria who was among the first military officers to publicly defect from Mr. Assad’s forces was killed in battle with his former colleagues, antigovernment activists and state news media said.
The leader, Yasser al-Abboud, had risen to the rank of lieutenant colonel in the Syrian Army before joining the uprising against Mr. Assad in 2011 and leading a major rebel formation in the southern province of Dara’a.
Antigovernment activists posted videos of the funeral procession in Mr. Abboud’s home village, where dozens of rebels fired their guns in the air as a final salute.
Even as the Obama administration has pointed to the growing role of extremists in Syria, its policy has continued to be a target for critics, who complain that the United States has offered the moderate Syrian opposition too little, too late.
American officials have announced no major new efforts to provide arms or other military support to the moderate opposition that it hoped would help counter the extremists.
Last month, officials notified Congress that the Obama administration would provide an additional $100 million in nonlethal assistance to the moderate opposition as part of a $250 million package that had been announced.
At a news conference in Paris, Mr. Kerry acknowledged that the military situation in Syria had shifted somewhat in Mr. Assad’s favor since Mr. Kerry and Sergey V. Lavrov, the Russian foreign minister, announced plans in May for a Geneva peace conference.
But Mr. Kerry insisted that the Syrian government’s military gains had not weakened the Obama administration’s diplomatic strategy to establish a transitional government in Syria that would not include Mr. Assad.
“It doesn’t matter whether you’re up or whether you’re down on the battlefield, the object of Geneva 2 remains the same,” Mr. Kerry said of the conference.
“If he thinks he’s going to solve problems by running for re-election I can say to him, I think, with certainty this war will not end as long as that’s the case or he is there,” Mr. Kerry said, referring to Mr. Assad.
In addition to planning for the Geneva conference, officials at the Tuesday meeting in London hoped to consider ways to strengthen the moderate rebels, who were expected to be represented at the meeting by Ahmad Assi al-Jarba, the head of the political wing of the Syria opposition coalition.
The other participants were to include senior diplomats from the “London 11” nations that have backed the moderate opposition: Britain, Egypt, France, Germany, Italy, Jordan, Qatar, Saudi Arabia, Turkey, the United Arab Emirates and United States.
The United States and five other world powers have been in talks with Iran on how to limit its nuclear program.
But Mr. Kerry said there was little reason to think Iran could play a helpful role at a Syria peace conference because the Iranian government had not formally agreed that the goal should be a transitional government that excludes Mr. Assad.
Mr. Kerry noted that Iran had sent arms and personnel to Syria to assist Mr. Assad, as had Hezbollah, the Lebanese militant group Iran supports.
“Hezbollah and Iran represent the two only outside organized forces in Syria fighting on behalf” of Mr. Assad, Mr. Kerry said. “So I think it’s time for the United Nations and for others to consider the appropriateness of their activity.”