MOSCOW (Reuters) - U.S. Secretary of State Rex Tillerson has given assurances that Washington s only goal in Syria is fighting Islamic State, the TASS news agency cited Russian Foreign Minister Sergei Lavrov as saying on Thursday. Lavrov said he had last had a telephone conversation with Tillerson on Oct. 9, the agency reported. Russia s defense ministry accused the United States on Tuesday of pretending to fight Islamic State and of deliberately reducing its air strikes in Iraq to allow the group s militants to stream into Syria to slow the Russian-backed advance of the Syrian army. Lavrov, touching upon a possible U.S. withdrawal from the Iran nuclear deal, said it was hard to imagine how it could be legally implemented, TASS reported. 