Areas of "difficulty and difference" remain between Russia and the US ahead of key talks on the crisis in Syria, a US official says.
The state department spokesman played down chances of a deal at the talks, to take place on Saturday in Geneva.
Russian Foreign Minister Sergei Lavrov earlier met US counterpart Hillary Clinton, and said there was a "very good chance" of finding common ground.
More than 100 people have been killed in Syria since Thursday, reports say.
At least 25 people, including 11 civilians and 13 soldiers, died on Friday, and there were 117 civilian deaths on Thursday, the UK-based Syrian Observatory for Human Rights said.
Other groups gave higher death tolls.
Meanwhile, Syrian President Bashar al-Assad said he would not accept any solution to his country's crisis imposed from outside.
He told Iranian television that it was an "internal issue" which had "nothing to do with foreign countries", stressing that no amount of foreign pressure would make his government change its policy on internal security.
'Change in position'
Saturday's conference in Geneva was called by the UN and Arab League envoy, Kofi Annan, as the violence intensified in Syria.
Mr Lavrov and Mrs Clinton met in St Petersburg in an effort to agree a consensus formula to end the bloodshed.
While Russia has been hostile to any solution that would see Mr Assad forced from power, the US has insisted he steps down.
After leaving the talks with Mrs Clinton, Mr Lavrov said: "We have a very good chance to find common ground at the conference in Geneva tomorrow.
"I felt a change in Hillary Clinton's position. There were not ultimatums. Not a word was said that the document we will discuss in Geneva is untouchable."
But a US state department official later told reporters: "There are still areas of difficulty and difference."
Discussing the chances of an agreement being reached on Saturday, the official said: "We may get there, we may not."
Meanwhile, Mr Lavrov's deputy Gennady Gatilov said on his Twitter account: "Our Western partners want to decide the outcome of the political process in Syria themselves, even though it is a job for the Syrians."
Those invited for the Geneva talks include the five permanent members of the UN Security Council, as well as Turkey, Iraq, Kuwait and Qatar.
Mr Annan wants support for an interim government that could include opposition members and officials serving under Mr Assad, but exclude those "whose continued presence and participation would undermine the credibility of the transition and jeopardise stability and reconciliation", his spokesman said.
Diplomats said this was an implicit reference to the Syrian president.
Syrian opposition groups have said the president would have to hand over power and leave the country as part of any settlement.