AP - Four Syrian nuclear scientists were killed near Damascus after gunmen opened fire on their bus in an area where there was no fighting, suggesting they were the target of the attack, according to activists and a pro-government website.

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 communities. Al-Shami said the area tended to be quiet because of locally-negotiated truces. Al-Shami and the Damas Now site said Syrian troops sealed off the nearby town of al-Tal following the attack.

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

An Israeli air strike 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.

Also Monday, UN envoy to Syria Staffan de Mistura held talks with President Bashar Assad and headed to the central city of Homs, said an official in his delegation, who spoke on condition of anonymity because he wasn't authorized to speak to media.

The visit came after Al-Watan on Saturday accused de Mistura of overstepping his bounds as a UN diplomat by publicly raising the idea of conducting more negotiated truces.

Al-Watan said de Mistura will meet in Homs 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.