The talks are already being boycotted by the Syrian opposition and the Kurds.
The Syrian Dialogue Congress is expected to gather more than 1,500 representatives of Syrian society: opposition and pro-government forces, as well as representatives of Syria's numerous ethnic and religious groups and the delegations from states participating in the peace process, with the United Nations and several regional and worldwide organizations present as observers.
He added however, that under-representation would not "disrupt this congress or undermine its importance", calling the Sochi talks a "very important" step toward peace.
Paris and London will not be taking part in Sochi, saying the talks had to be part of a UN-led process and urging Moscow to get the Syrian regime to engage in meaningful negotiations.
Kremlin spokesman Dmitry Peskov said on a conference call with reporters: "The fact that some representatives of the processes now taking place in Syria are not participating is unlikely to stop this congress from going ahead and is unlikely to seriously undermine the importance of the congress".
Ukrainian citizens not injured in Kabul explosion on Saturday
It was the second successful Taliban attack in a week on high security targets in the city. This year, they insist, the Taliban will begin to lose territory again.
Some 1,600 Syrian politicians, rebels and members of civil society will attend the negotiations in the Black Sea Resort, which follow another round of failed United Nations peace talks in Vienna. The first group of the congress participants arrived in Sochi yesterday.
The department issued a statement last week asserting that Mr. Tillerson had told Russian Foreign Minister Sergey Lavrov via telephone that Washington hopes Russia will play a "constructive role" in the Geneva process, regardless of the alternative talks being orchestrated by Moscow.
"We urge Russian Federation to use its influence to persuade the regime to cease its destructive behavior".
Regime-backer Moscow has invited 1,600 people to the talks in the Black Sea resort of Sochi as part of a broader push to consolidate its influence in the region and start hammering out a path to a political solution to end the bloody conflict. France's Syria envoy is not due to go, a diplomatic source said.
Kremlin spokesman Dmitry Peskov said on a conference call with reporters Monday that the opposition's boycott would not seriously hinder the conference, according to Reuters.