The United States has warned that “a lot of work” remains to ensure that the upcoming Syria talks will be held next week as planned.
Washington is urging the participants in the United Nations-sponsored dialogue to move the process forward, despite differences over which militant groups should be labeled “terrorist” and who will get to represent the opposition.
Seeking to break the impasse, Secretary of State John Kerry is scheduled to meet with his Russian counterpart Sergei Lavrov in Zurich on Wednesday.
The top US and Russian diplomats will attempt to strike a deal over who should be allowed to sit at the negotiating table in the Geneva talks slated to begin on Monday.
President Barack Obama and Vladimir Putin, his Russian counterpart, failed to agree on a list in a telephone conversation last week.
"The secretary (Kerry) will certainly talk about Syria and our ongoing efforts to get a political transition in place with Foreign Minister Lavrov," State Department spokesman John Kirby said on Tuesday.
"It is still our desire to see this meeting occur on the 25th," he said, adding that Kerry is also in regular contact with the UN envoy for Syria, Staffan de Mistura.
The United Nations also hinted at a possible delay and de Mistura has yet to send out formal invitations to the talks.
"We're not unmindful of the fact that there still remain differences of opinion, and that this is a complicated process," Kirby said.
"And that there is still quite a bit of work that needs to be done to get the meeting to occur," he added.
In Moscow, meanwhile, a senior Russian diplomat expressed optimism that the meeting between Lavrov and Kerry would produce an agreement on the list of eligible opposition figures.
While Washington still demands that President Bashar al-Assad must step down, it has backed away from insisting that the Syrian leader go at the start of a transition process.
Some 400,000 people in Syria are under siege as a foreign-backed militancy is wreaking havoc across much of the country.