Thirteen Syrian opposition activists were killed during a daring and chaotic attempt to smuggle four foreign journalists out of a besieged Homs neighborhood, human rights groups said Tuesday, illuminating the risks taken by the networks that routinely ferry injured people out of stricken Syrian cities.

Syrian security forces ambushed the group, which included about 40 activists as well as the four journalists, overnight Monday as they tried to leave Baba Amr, according to the advocacy group Avaaz, which helped coordinate the rescue attempt. The neighborhood has been a center of resistance to President Bashar al-Assad and the main target of his government’s efforts to quell the nearly year-long revolt.

One of the journalists, injured British photographer Paul Conroy, managed to escape and was whisked across the border to Lebanon early Tuesday after three of those accompanying him were killed, according to Avaaz.

But the ferocity of the attack scattered the rest of the group, Avaaz said. The three other journalists were forced to turn back and remain trapped in Baba Amr. Ten more activists escorting them were killed in the confused retreat into the neighborhood, which has come under sustained bombardment for the past 25 days.

The 13 were among 102 people killed across Syria on Tuesday, according to the Local Coordination Committees, a Syrian activist group. The toll included 26 who died in Baba Amr as the government continued its artillery and rocket attacks against the opposition stronghold.

In Geneva, the United Nations’ political officer, B. Lynn Pascoe, said that “well over” 7,500 Syrians are now known to have been killed since the uprising began last March 15, and that an average of 100 are dying daily as the Syrian government attempts to suppress what has become the bloodiest and most intractable of all the Arab revolts.

The plight of the trapped journalists, two of whom were injured in a rocket attack on the house where they were staying last Wednesday, focused attention on the perils of reporting on the escalating violence in Syria at a time when the authorities’ refusal to grant visas has prompted some journalists to sneak into the country illegally.

American correspondent Marie Colvin, who worked for Britain’s Sunday Times, and French photographer Remi Ochlik were killed during the bombardment last Wednesday, which came days after New York Times correspondent Anthony Shadid died of an asthma attack during a strenuous trek out of Syria across the mountainous Turkish border.

The failed rescue Tuesday also underscores the dangers facing the underground networks of activists and smugglers set up to evacuate people injured in government attacks to hospitals in Lebanon, Turkey and Jordan. The same networks carry medical supplies such as blood bags and antibiotics into Syria for use in field hospitals and have been used by journalists to enter the country illegally.

Similar but separate networks also have been utilized to smuggle weapons to the fledgling armed resistance movement known as the Free Syrian Army. But most of those active in the medical networks are civilian volunteers, seeking to help Syrians who have been injured during protests and who risk detention if they seek treatment at government hospitals, said Wissam Tarif, a Lebanon-based activist with Avaaz.

“They are just ordinary guys who did not pick up weapons but decided to evacuate injured people,” he said. “Some of them have basic medical training, some can do tetanus shots and provide some medical assistance. Some of them are just guys who can carry heavy weights. They’ve been doing this for a year, and hundreds of them have been killed.”

Tarif, who has close ties with the network, said 23 members engaged in ferrying medical supplies and injured victims between Homs and Lebanon have been killed since last Wednesday’s attack on the journalists. In the process, they have evacuated 40 injured civilians from Homs.

Details of the the ambush and the identities of the dead Syrians were not disclosed to protect future evacuation operations.

With the deaths of the activists and the evident discovery of the secret route they had been using by Syrian security forces, the network is now in jeopardy, activists said, leaving it unclear whether the remaining journalists can be evacuated. Efforts by the International Committee of the Red Cross and the Syrian Arab Red Crescent to negotiate safe passage out of Baba Amr for the journalists have failed.