Senior American and Russian diplomats held "serious discussions" in Moscow on Syria, the US state department said on Thursday.

The talks were held between US Under-secretary of State for political affairs Wendy Sherman and Russia's two deputy foreign ministers, Gennady Gatilov and Sergei Ryabkov, said State Department spokeswoman Victoria Nuland in a statement, describing the discussions as "direct and fulsome."

During the talks, Sherman indicated that Washington did not support extending the UN observer mission in Syria, which was set up in April, as the cease-fire no longer existed in the Middle East country, Nuland said.

"The beginning of a political process is in the international community's long-term interests," she added.

The talks also involved Iran, during which Sherman reiterated the US commitment to the two-track approach on the nuclear stalemate, believing "diplomacy still has a chance to succeed," according to Nuland.

The United States and Russia have differed on how to deal with the 17-month-old crisis in Syria.

According to the Russian Foreign Ministry, Deputy Foreign Minister Gatilov stressed the need to maintain the UN presence in Syria during his meeting with Sherman, warning of serious consequences if the United Nations pulls out.

The mandate of the UN observer mission in Syria was not extended at the UN Security Council's meeting on Thursday. However, the council agreed to establish a new civilian office in Damascus to back the mediation efforts of the United Nations and the Arab League.