DAMASCUS, Syria — Syrian President Bashar Assad said a cease-fire proposal for the embattled northern city of Aleppo, which was raised by the United Nations envoy to the country, was "worthy of studying," in remarks made to state-run media Monday.

Also Monday, activists and a pro-government website reported that four Syrian nuclear scientists were killed near Damascus. The men were killed when gunmen opened fire on their bus in an area where there was no fighting, suggesting they were the target of the attack.

It wasn't immediately clear if Assad's remarks about a cease-fire in Aleppo reflected a change in the government's stance. But it came after a Syrian newspaper that acts as a government mouthpiece, recently accused the U.N. envoy, Staffan de Mistura, of overstepping his bounds by publicly raising the idea of small-scale, localized and negotiated truces.

De Mistura and Assad held talks earlier Monday, where the U.N. envoy introduced the idea of local cease-fires in Aleppo, the last major city where rebels still hold large areas as they battle government forces.

"President Assad ... considered that the initiative of de Mistura was worthy of studying, and of working on, to reach its target of returning security to the city of Aleppo," said the statement published by the state-run SANA news agency.

The U.N. envoy also headed to the central city of Homs on Monday, where he was expected to meet with a delegation representing armed groups from al-Waar, the last rebel-held part of the city.

The international diplomat is on a three-day visit to Syria aimed at reducing the violence. Syria's conflict has claimed an estimated 200,000 lives since the uprising against Assad began in March 2011.

Meanwhile, at least four nuclear scientists were killed when gunmen opened fire on their bus, reported activists and a pro-government website on Monday.

Rami Abdurrahman of the Syrian Observatory for Human Rights, a Britain-based group that relies on activists in Syria, said an Iranian nuclear scientist was also killed in the attack. The pro-government website "Damas Now" also said a fifth person was killed, but that the person's identity was unknown.

The pro-government newspaper Al-Watan said only that four "nuclear scientists and electrical engineers" were killed.

Al-Watan said the men were on a bus heading to the Scientific Research Center near the Syrian capital when they were attacked on Sunday, and suggested the al-Qaida-linked Nusra Front was behind the assault.

Abdurrahman said there was no fighting in the area where the men were killed, near a bridge on a highway just north of Damascus. "There were no clashes there at all. It was an operation to assassinate them," said Abdurrahman.

Another Syrian activist, who goes by the name Abu Akram al-Shami, also said there were no clashes in the area — which is a mix of rebel and government-held neighborhoods. Al-Shami said the area tended to be quiet because of locally-negotiated truces.

Syrian facilities suspected of being used for military and nuclear research have been targeted in the past.

An Israeli airstrike struck a military and scientific research center near Damascus last May. The nature of the research center was unclear. In January, Syrian officials accused Israel of striking another scientific research center northwest of Damascus.

In 2007, Israel bombed a suspected Syrian nuclear reactor — an attack confirmed by U.S. officials. Israel has never commented on the incident.

Syrian, Iranian and Israeli officials could not immediately be reached for comment on the killing of the scientists.

On Oct. 30, de Mistura said at the United Nations headquarters in New York that he was pursuing an "action plan" that involves freezing the conflict in certain areas to allow for humanitarian aid and local steps toward a political process that would allow wider peace. The diplomat also stressed that the Geneva communique, agreed on by major powers at a conference in June 2012 calling for the establishment of a transitional governing body for Syria, was still valid — even though Assad was elected this year to another seven-year term.

De Mistura's presentation drew an immediate backlash from Syrian media considered mouthpieces for the Assad government; the daily Al-Thawra newspaper warned that veteran diplomat took a "hasty" attitude in his briefing and charged that he deviated from the "limits of the international mission" with which he was entrusted.

Hadid reported from Beirut.