MOSCOW—Russian Foreign Minister Sergei Lavrov said Iran would "inevitably" take part in any resolution to the civil war in Syria, as the Iranian and Syrian foreign ministers held a round of meetings with Russian officials in Moscow.

After a meeting with Iranian Foreign Minister Javad Zarif, Mr. Lavrov said Iran should take part in an international peace conference aimed at finding a diplomatic solution to the war.

"We are proceeding on the basis that Iran should and inevitably will be part of a set of measures to settle the Syria problems," Mr. Lavrov told reporters.

Iran, an ally along with Russia of Syrian President Bashar al-Assad, hasn't been invited to peace talks scheduled for Switzerland next week aimed at finding a diplomatic resolution to the conflict. Mr. Lavrov said that Russia, Syria and Iran had no "hidden agenda" and didn't have a tripartite peace draft.

In Washington on Thursday, U.S. Secretary of State John Kerry urged the Syrian opposition coalition to attend the talks, reiterating that any names put forward to be part of a transitional government must be acceptable to both sides. "This means that any figure that is deemed unacceptable by either side, whether President Assad or a member of the opposition, cannot be a part of the future," Mr. Kerry said.