The United States said on Tuesday that it "doesn't seem to make sense" that Iran was holding Washington responsible for the lives of 48 Iranians abducted in Syria.

"At this point, we're still following the reports closely, monitoring the situation. We can not confirm the identity of those reportedly kidnapped," State Department spokesman Patrick Ventrell said at a regular news briefing.

"We don't have... information about who these people are, where they are," he added, calling on "anyone holding prisoners in this conflict to treat them humanely, in accordance with international law."

The group of Iranians were taken hostage on Saturday by militants in the Syrian capital of Damascus. Iran says they were Shiite pilgrims, not Revolutionary Guards as the rebels have claimed.

Press reports said on Monday that three of the abductees had been killed and Syrian rebels had threatened to kill the others if the Syrian army does not stop its attack.

Iran's Foreign Ministry on Tuesday summoned the charge d' affaires of the Swiss embassy, which represents the U.S. interests in Iran since Tehran and Washington severed diplomatic relations in 1980, to protest against the abduction, Iran's Press TV said.

The ministry said Washington is responsible for the lives of the Iranian nationals and should use its influence on the opposition groups in Syria to secure their release "without any preconditions," the report said.

Tehran said Turkey and Qatar, which are supporting opposition groups in Syria, are responsible as well for the lives of the kidnapped Iranians.