Israel has reliable information showing that despite Syria's official denials, an attempt was made to assassinate several top regime officials four days ago, senior Israeli officials said on Thursday.

The information shows that Syrian President Bashar Assad's brother-in-law, Assef Shawkat, and several other senior officials were indeed poisoned, just as the Free Syrian Army claims. But prompt medical treatment saved their lives.

Israel views this as an important milestone in the ongoing revolution: It shows the opposition has penetrated Assad's inner circle, casting doubt on the regime's stability.

"The Syrian revolt is no longer in faraway cities; it's reached the leaders of the regime," one senior Israeli official said. "An assassination on a similar scale in the future could accelerate the regime's collapse. The opposition has the tools to reach the heads of Assad's regime, and in this case, it's been proven."

The Israeli officials, who are well-briefed on intelligence from Syria, said the poisoning was carried out by the Free Syrian Army's Al-Sahaba battalion. The targets, aside from Shawkat, were Defense Minister Daoud Rajha; Interior Minister Mohammed al-Shaar; Hassan Turkmani, an aide to the vice president; national security chief Hisham Bakhtiar; and Mohammed Said Bakhtian, national secretary of the Baath Party.

On May 20, the Free Syrian Army announced that an assassination attempt had been carried out the previous evening by Bakhtian's bodyguard, who was recruited into the Syrian opposition. The bodyguard, it said, poisoned the food served to senior regime officials during a meeting of the "crisis management cell," charged with managing the ongoing fighting in Syria. As a result, the announcement added, the officials were rushed to the hospital during the meeting.

In response to this announcement, some of those who attended the meeting either made rare appearances on Syrian television or phoned the studio during a live broadcast to prove they were still alive.

Al-Shaar denied that the attempted assassination ever took place. "I'm calling you right now from my office," he said during a live television broadcast. Turkmeni also phoned the studio.

But other senior officials who attended the meeting have not been seen publicly since that day.

A senior Israeli official said the information obtained indicates that the food at the meeting really was poisoned. "All those who ate the food were rushed to the hospital and saved at the last minute by medical attendants," he said. "The bodyguard who slipped the poison in was smuggled out of Syria."

Another Israeli official said the information published by the Syrian opposition after the incident is deemed credible by Israel. "There was an attempt to poison Shawkat and the other senior officials, but it failed, and all those who were at the meeting are still alive," he said.