The United States has new evidence that Iran and Hezbollah have direct involvement with the Syrian regime, a senior State Department official told reporters traveling with Secretary of State John Kerry in Oman.

The official said that, according to the Free Syrian Army, Hezbollah and Iranian fighters have been helping the Syrian regime of President Bashar Assad in Qusayr, near the opposition stronghold city of Homs.

"It is the most visible effort we have seen of Hezbollah to engage directly in the fighting in Syria as a foreign force. We understand there are also Iranians up there," the official said. "This is an important thing to note - the direct implication of foreigners fighting on Syrian soil now for the regime."

The official said there are concerns that if the Syrian forces capture Qusayr they will slaughter the civilian population there, which numbers in the thousands. The opposition warns it could be a repeat of the massacres seen in Banias earlier this month, which is roughly 30 minutes away.

However, the official could not verify exactly what the Iranians and Hezbollah are doing - whether they are fighting alongside the regime or just advising the soldiers.

" I don't think they're arming because I've not heard that, but I think they could be doing a little of both advising and fighting," the official said. "We know that Iran and Hezbollah cooperate in a number of countries, not just in Syria. And so it is not a surprise that Iran would be there with Hezbollah on the ground. We do have consistent reports of Hezbollah fighters on the ground."

Meanwhile in Washington, the Senate Foreign Relations Committee approved the bi-partisan Syria Transition Support Act, which approves lethal aid and training to vetted Syrian rebels, sanctions weapons and oil sales to the Assad regime, and provides further humanitarian assistance for planning for a post-Assad Syria. All but three senators on the committee voting for the bill, which will now go to the full Senate for a vote.

The legislation allows for the U.S. to provide rebels with arms and military training only after they have gone through a vetting process by the U.S. government and found to meet human rights, terrorism and non-proliferation criteria. The bill also creates a $250 million transition fund for the next two years to help Syria's political opposition transition to governing the country, including supporting new institutions and supporting government institutions that currently exist.

While most of the committee members strongly supported the bill sponsored by chairman Sen. Robert Menendez, D-N.J., and ranking member Sen. Bob Corker, R-Tenn., of the three senators who opposed, Rand Paul, R-Ky., was the most vocal, calling the Syria conflict "murky" and warning that America was getting involved in a situation "where it's impossible to know who are friends are."

Paul cited the current problem with insider attacks in Afghanistan as an example of how, when not careful, the United States leaves itself vulnerable to attack by the very people it is trying to help.

"Syria is 100 times messier than Afghanistan," Paul said, and warned that the measure could be "a slippery slope to war."

Sen. Tom Udall, D-N.M., also expressed skepticism of the act, saying that he doesn't think the United States knows whom they are really arming.

But Senators Marco Rubio, R-Fla.; John McCain, R-Ariz.; Corker, and Menendez passionately argued that if the United States doesn't do anything now, the only people with weapons will be the Assad regime and the extremist elements of the opposition.

"Extremists groups with links to al Qaeda are exploiting the conflict and gaining ground in a state with large chemical weapon stockpiles," Menendez said. " The time to act and turn the tide against Assad is now."