WASHINGTON (AP) — Syrian rebels battling the forces of President Bashar Assad must receive ammunition and heavy weapons to counter the regime's tanks and aircraft or it will be impossible for them to prevail, Sen. John McCain said days after he quietly slipped into Syria to meet with the opposition.

"They just can't fight tanks with AK-47s," McCain said Friday in a telephone interview.

The Republican lawmaker and 2008 presidential candidate made an unannounced visit to Syria on Monday, traveling across the border near Kilis, Turkey, and spending about two hours meeting with rebel leaders. McCain has been one of the most vocal lawmakers demanding aggressive U.S. military action in the 2-year-old Syrian civil war, calling for establishment of a no-fly zone and arming the rebels.

The Obama administration has been reluctant to provide weapons to the disparate opposition, fearing that they will fall into the wrong hands in a volatile region. McCain said he discussed what types of weapons the rebels need and whether they could ensure their control.

"I'm confident that they could get the weapons into the right hands and there's no doubt that they need some kind of capability to reverse the battlefield situation, which right now is in favor of Assad," McCain said.

McCain, a member of the Armed Services and Foreign Relations committees, was the first U.S. senator to travel to Syria since the civil war began more than two years ago. He said he worked with Deputy Secretary of State William Burns in arranging the trip.

McCain said he spoke with Secretary of State John Kerry "a couple of times. It wasn't that I was hiding it from him; it just didn't seem to come up. I thought Burns was the right guy to go through. They were very important in the trip. We couldn't have done it without their cooperation."

Gen. Salim Idris, chief of the Supreme Military Council of the Free Syrian Army, accompanied McCain and they met with 19 battalion commanders.

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Citing the photo of McCain's meeting, a Lebanese newspaper has reported that McCain unwittingly crossed paths with two men connected to a rebel group responsible for the kidnapping of 11 Lebanese Shiite pilgrims in 2012. McCain said one of the men he reportedly met with is dead and no one in his meeting was identified as the other.

"The people I met with and talked to directly were well-vetted. Their names and their duties were outlined to me. They came from all over Syria," he said.

Two years of violence in Syria have killed more than 70,000 people, according to the United Nations. President Barack Obama has demanded that Assad give up power, while Russia has stood by Syria, its closest ally in the Arab world. Russian officials have said they will support anti-aircraft systems to Syria, and Assad suggested on Thursday that he had received the first shipment.

The United States and Russia are trying to get the Syrian government and opposition forces into peace negotiations. Those talks, initially planned for Geneva next month, have been delayed until July at the earliest.

"It's hard to imagine Bashar Assad negotiating his departure when he has the upper hand on the battlefield," McCain said. "I'm all for a conference, but I think that conference should take place when Bashar Assad knows that he is doomed to defeat if he doesn't negotiate."

Last week, the Senate Foreign Relations Committee voted to provide weapons to rebels in Syria, as well as military training to vetted rebel groups and sanctions against anyone who sells oil or transfers arms to the Assad regime.

The European Union decided late Monday to lift the arms embargo on the Syrian opposition while maintaining all other sanctions against Assad's regime after June 1, British Foreign Secretary William Hague said.