The United States says it has no information about Iranians kidnapped in Syria and has criticized Tehran's increasing support for the regime of Syrian President Bashar Assad.

"We are still following the reports closely and monitoring the situation. We cannot confirm the identity of those reported to be kidnapped, and the wider issue of us having deep concerns about Tehran's destructive behavior in Syria continues, but these particular individuals, we do not have any independent information about who they are," said State Department acting deputy spokesperson Patrick Ventrell in a press briefing, late on Tuesday.

"I can confirm that the Iranians called in our Swiss protecting power - but I understand we did not receive any official correspondence through that channel," he added, indicating at diplomatic efforts by Tehran at various levels to try win release of the Iranians.

Some 48 Iranians had been taken as hostages by gunmen of the Syrian opposition on outskirts of the Syrian capital. The opposition accused them of being affiliated with the Iranian Revolutionary Guards, but Tehran denied this and said the Iranians were on a religious trip to a sacred site.

Ventrell talked about the Iranian role in Syria, saying that "what we have seen in the last weeks is that Tehran has intensified its relationship with Assad's Government - high-level visits, signing economic agreements, passing a bill in parliament to cement its support for the Syrian Government". "It is just unconscionable that the Iranian Government is ignoring the massacres of civilians in Aleppo and throughout Syria, and instead, finding new ways to try and prop up a regime who is killing many of thousands of its own citizens. So that's sort of our broader opinion about Tehran's role in Syria," he added.

Ventrell remarked that the Syrian opposition is "gaining ground, and there is going to be a new day in Syria and obviously, our diplomacy is intensive in the sense that we are working with a hundred-plus other countries who are on the same page that we are to push the opposition to become more united to have a plan for the day after to run a new government." "What we want to see is the quickest possible transition to avoid further bloodshed. What we are looking for is the quickest way to get to a peaceful solution," he added.

Ventrell said that there is no doubt that "the slaughter continues, and it is despicable" in Syria and some of the methods being used "like fixed-wing aircraft, it is particularly reprehensible.

There is no doubt that the conflict continues." "But the opposition, in the face of this onslaught - and maybe Assad expected them to just be suppressed and stopped, but the will of the Syrian people has not been crushed. They continue to press on and continue to have remarkable gains," he concluded.