The U.S. government on Thursday admitted hackers accessed the personal data of at least four million current and former federal employees, in a vast cyber-attack suspected to have originated in China.

"As a result of the incident," uncovered in April, the Office of Personnel Management said it "will send notifications to approximately four million individuals."

Thousands of Iranian and Iraqi fighters have been deployed in Syria in past weeks to bolster the defenses of Damascus and its surroundings, a Syrian security source told AFP on Wednesday.

"Around 7,000 Iranian and Iraqi fighters have arrived in Syria over the past few weeks and their first priority is the defense of the capital. The larger contingent is Iraqi," the source said on condition of anonymity.

Iranian pilgrims at a Shiite holy site near the city of Qom intervened against the distribution of leaflets opposing Tehran's nuclear talks with world powers, state news agency IRNA reported Wednesday.

It said activists linked to the students' branch of the basij, Iran's Islamist volunteer militia, distributed the leaflets outside the Jamkaran mosque near the city in central Iran when several pilgrims stepped in to oppose their action.

Globe-trotting U.S. diplomat John Kerry on Tuesday underwent successful surgery to mend a broken leg, and his doctor hopes to get him up and walking within 24 hours.

"The procedure was uncomplicated, the fracture was fully repaired, and we plan to get him up walking on Wednesday," Kerry's orthopedic surgeon Dennis Burke, from Massachusetts General Hospital, said in a statement.

U.S. lawmakers Tuesday called on Tehran to immediately free four Americans held or missing in Iran after poignant testimony from relatives who described "living a nightmare" with little news and fading hope.

Some on the House foreign affairs committee and some family members urged the U.S. administration to halt nuclear talks with the Islamic republic until the four are freed, or to make any deal contingent on their release.

A bus carrying Iraqi tourists plunged off a mountain road into a valley in northern Iran Tuesday, killing 24 holidaymakers and injuring 16 others, Iranian state television said.

The accident occurred between Tehran and the Caspian Sea in the Alborz mountains. 

An American among several believed held by Huthis in Yemen was freed and receiving treatment Tuesday in neighboring Oman, which has been hosting talks between Washington and the Iran-backed Shiite rebels.

A diplomat in Muscat said the talks between the Americans and the Huthis were aimed at promoting a proposed peace conference in Geneva, which the United Nations has so far failed to persuade the warring parties to attend.

U.S. top diplomat John Kerry remains focused on nailing down a nuclear deal with Iran and plans to join the final negotiations in late June despite breaking his leg in a cycling accident, officials said Monday.

The globe-trotting Kerry landed in Boston from Geneva on a military C-17 plane accompanied by his personal doctor late Monday.

U.S. President Barack Obama said a negotiated deal and not military action is the way to prevent Iran procuring nuclear weapons, in an interview broadcast on Israeli television on Monday.

"I can, I think, demonstrate, not based on any hope but on facts and evidence and analysis, that the best way to prevent Iran from having a nuclear weapon is a verifiable tough agreement," he told 2, a private station.

A nuclear deal with Iran will significantly roll back Tehran's military capability, U.S. Vice President Joe Biden's national security adviser told a conference in Qatar on Monday.

Dr. Colin H. Kahl, speaking at the U.S.-Islamic World Forum in Doha, said the current deal being hammered out was the best on offer, despite many skeptical voices in the U.S. and elsewhere, including Gulf Arab states.