Talks on a protracted row over Iran's nuclear ambitions resume this week in Geneva, with Tehran announcing a visit by the U.N. atomic watchdog chief to try to clinch a deal.

The so-called P5+1 group of major powers will meet Iran's nuclear team on Thursday and Friday for the latest round of negotiations revived after the election of Iranian President Hassan Rouhani, a reputed moderate.

Russia insisted on Tuesday that Iran must be invited to peace negotiations on Syria that world powers hope to hold in Geneva later this month.

"All those who affect the situation must be invited to the conference," Russian Foreign Minister Sergei Lavrov told reporters.

An Iranian commander of the Revolutionary Guards was killed in Syria by "terrorists" while defending "oppressed Syrian people" near Damascus, media reports said Monday.

Iran -- a key ally of President Bashar Assad's embattled regime -- has repeatedly denied it has troops fighting in Syria.

Iran's President Hassan Rouhani is not "optimistic" about ongoing nuclear negotiations with world powers, the official IRNA news agency reported Monday ahead of a new round of talks this week.

"The government is not optimistic about the Westerners and the current negotiations," he was quoted as saying, echoing similar comments by supreme leader Ayatollah Ali Khamenei Sunday.

Thousands of Iranians shouted "Death to America" as they rallied Monday on the anniversary of the 1979 U.S. embassy takeover, in a setback to hopes for a thaw in relations.

Hardline and conservative factions had called for a massive demonstration 34 years after Islamist students stormed the embassy compound in Tehran, holding 52 American diplomats hostage for 444 days.

Top U.S. diplomat John Kerry traveled to Saudi Arabia on Sunday hoping to soothe tensions over Washington's refusal to intervene in Syria and its diplomatic overtures to Iran.

Relations with the longtime ally have been strained over U.S. reluctance to strike Syria or provide more aid to rebels there, as well as its tentative detente with Iran following the election of President Hassan Rouhani, a reputed moderate.

Iran on Monday marks the anniversary of the 1979 U.S. embassy takeover with its customary "Death to America" chants despite a taboo-breaking telephone call between their presidents.

Islamist students stormed the embassy compound in Tehran 34 years ago, holding 52 U.S. diplomats hostage for 444 days, rupturing diplomatic relations and provoking decades of mutual hostility.

A Manama court Sunday jailed four Shiites for life and sentenced six others to 15-year terms for forming an Iran-linked "terrorist cell" to mount attacks in Bahrain, a judicial source said.

The defendants were part of a group of 24 on trial, of whom 14 were acquitted.

The head of the U.N. atomic watchdog said Friday that Iran's new leadership has indicated a willingness to address concerns regardless of progress in parallel talks with world powers.

The International Atomic Energy Agency said this week that it held "productive" talks with President Hassan Rouhani's government, which come as Iran conducts separate negotiations with the United States and five other nations over its nuclear program.

U.S. Vice President Joe Biden and other top administration officials met with senior Democrats in a bid to persuade the Senate to hold off on new Iran sanctions, his office said Friday.

With Washington negotiating with Tehran's new leadership over the Islamic republic's nuclear drive, Biden sought to press pause -- at least for now -- on efforts by Congress to pass even tighter sanctions, to allow the talks to progress.