TEHRAN — Iran said Tuesday that it was holding the United States responsible for the fate of a group of Iranians held by Syrian rebels, as the highest-ranking Iranian official to visit Syria since the antigovernment uprising began there arrived in Damascus to show support for President Bashar al-Assad and attempt to secure the release of the hostages.

The warning by Iran, Syria’s last remaining friendly state government in the region, came after three of the 48 hostages were reported killed during an artillery attack on rebel positions by the Syrian Army on Monday and their captors threatened to kill the rest if the shelling did not stop.

The rebels, who released a video of some of the abducted Iranians on Sunday, said the captives were members of Iran’s Revolutionary Guards Corps who had been heading to the airport after a reconnaissance mission in Syria. Iran said the men were part of a group of religious pilgrims, including women and children, who had visited the shrine of the granddaughter of the prophet near Damascus.

In an unusual move, Iran’s Foreign Ministry announced it had sent a diplomatic note to the Obama administration saying that “due to its open support for Syrian terrorist groups” the United States was responsible for the safety of the abducted Iranians.