DUBLIN -- A new round of diplomacy on the conflict in Syria will begin on Thursday afternoon when Lakhdar Brahimi, the United Nations special envoy, hosts an unusual three-way meeting with Secretary of State Hillary Rodham Clinton and the Russian foreign minister, Sergey V. Lavrov.
The session, which is being held on the margins of a meeting on European security, comes amid reports of heightened activity at Syria's chemical weapons sites and signs that Russia may be shifting its position on a political transition in Syria.
"Secretary Clinton has accepted an invitation by U.N. Special Envoy Brahimi for a trilateral meeting on Syria this afternoon with Mr. Brahimi and Russian Foreign Minister Lavrov," a senior State Department official said Thursday morning.
This is not the first time that American and Russian consultations have spurred hopes of a possible breakthrough. In June, Mrs. Clinton, Mr. Lavrov and the United Nations's envoy on the Syrian crisis at the time, former United Nations Secretary General Kofi Annan, appeared to be close to an agreement that a transitional government should be established and that President Bashar al-Assad give up power.
But that seeming understanding quickly broke down, with American officials complaining privately that the Russian side had pulled back from the deal. A major sticking point, it later emerged, was the American insistence that the United Nations Security Council authorize steps to pressure Mr. Assad if he refused to go along under Chapter 7 of the United Nations Charter, which could be used to authorize tougher economic sanctions and, in theory, the use of force.
It remained to be seen if the new round of negotiations would be more successful.
On the one hand, the military situation on the ground appears to be shifting in the rebels' favor. Some Russian officials reportedly no longer believe that Mr. Assad will succeed in holding on to power and may have a new interest in working out arrangements for a transition. The changing battlefield, some experts say, may have led to a softening of the Russian position.
A senior Turkish official said that after President Vladimir V. Putin of Russia and Prime Minister Recep Tayyip Erdogan of Turkey recently met in Istanbul that Moscow was "softening" its "political tone" and would look for ways of getting Mr. Assad to relinquish power.
On the other hand, it was possible that Mr. Lavrov had, in effect, merely agreed to meet so that Russia could maintain influence over the discussions on Syria and find out what exactly Mr. Brahimi was prepared to propose.
There were indications on Thursday that Russian officials see the positions of Washington and Moscow on Syria moving slightly closer.
Deputy Foreign Minister Gennady Gatilov of Russia expressed satisfaction in a Twitter message that the United States was moving to designate Al Nusra Front, a Syrian opposition group seen by American experts as linked to Al Qaeda, as an international terrorist organization.
The aim of the American move, which is expected soon, would be to isolate radical foes of the Assad government.
With the rebels making gains on the ground, American officials have been trying to ensure that military developments do not outpace political arrangements for a possible transition. American officials have hinted that the United States would upgrade relations with the Syria opposition, possibly to formal recognition, if the coalition made progress on a political structure by the time of a meeting of the so-called Friends of Syria in Morocco.
But emerging policy on the Al Nusra Front also acknowledges Russia's longstanding argument that the Syrian opposition includes radical jihadists. Mr. Gatilov said that the American step "reflects understanding of the danger of escalating terrorist activity in Syria."
A lawmaker with the dominant party, United Russia, told British legislators visiting Moscow that Russia saw Mr. Assad's government struggling. "We think that the Syrian government should execute its functions," he said, according to the Interfax news service. "But time shows that this task is beyond its strength."
Dimitri K. Simes, a Russia expert the Center for the National Interest in Washington, said, based on conversations with top officials, that Russia has indeed softened its position in light of military setbacks for the Assad government, and it is now understood that neither Mr. Assad nor his close associates would take a central role in a new government.
However, he said Russia still wanted Iran to take part in negotiations about the transition. Iran's presence, he said, would reassure Alawites, the Shiite Muslim minority of Mr. Assad and the core of the military, that they would be protected in the change of government.
But even as there were hints that American and Russian stances might be converging, they differed sharply on human rights issues at the conference of the Organization for Security and Economic Cooperation in Europe, which began on Thursday.
Before the conference, Mrs. Clinton met in a tent outside the conference center with a group of 11 civil society representatives from seven countries, including Russia and nations from Eastern Europe and Central Asia.
Olga Zakharova, of Freedom Files in Russia, said she has worked for 20 years as a journalist and has seen her country "go from bad to worse."
"Even social media space is now shrinking," she said, citing new restrictive laws on the use of the Internet. "We ask you not to leave Russia and other countries on their own," she said.
The concern is no longer the environment, but the safety of colleagues, she said.
Addressing the group, Mrs. Clinton said that she agreed there was a move to "re-Sovietize" the region. "We agree with your assessment that the space for civil society and the protection of human rights and fundamental freedoms is shrinking and governments are becoming much more aggressive in trying to stifle dissent, prevent the free expression and exchange of views," she said.
Mrs. Clinton referred to the meeting with the human right activists and said that the future of the organization was in jeopardy because of actions taken by authoritarian members.
Mrs. Clinton also singled out human rights abuses in Belarus, election abuses in Ukraine and restrictions on free expression in Tajikistan, Turkmenistan, Uzbekistan, and Kazakhstan, among other nations.
On Russia, Mrs. Clinton criticized legislation that would require journalists and officials from nongovernment organizations to register as "foreign agents" if they received financing from abroad and other restrictions on civil society.
Mr. Lavrov repeated the Russian complaint about "unilateral approaches" that were interfering with the work of the organization.
Michael R. Gordon reported from Dublin, and Ellen Barry from Moscow. Anne Barnard contributed reporting from Beirut, Lebanon.
This article originally appeared in The New York Times.