The United States should lead an international effort to protect key population centers in Syria through air strikes on President Bashar al-Assad's forces, US Senator John McCain said on Monday.

"The ultimate goal of air strikes should be to establish and defend safe havens in Syria, especially in the north, in which opposition forces can organize and plan their political and military activities against Assad," McCain, an influential Republican who lost the White House to Barack Obama in the 2008 presidential race, said in a Senate floor speech.

There was no immediate comment from the Obama administration on the comments by McCain, the first US senator to urge a U.S. military strike on Assad's forces.

The administration has so far resisted being pulled into the crossfire in Syria, where the United Nations says over 7,500 people have died in the Assad government's nearly year-long crackdown on protesters.

The White House did say that "the tragic situation is Syria" was one of the matters discussed on Monday in an Oval Office meeting between Obama and visiting Israeli Prime Minister Benjamin Netanyahu.

McCain has previously called for efforts to arm the Syrian opposition. But he said on Monday that the help Syrian rebels needed most urgently was "relief from Assad's tank and artillery sieges in the many cities that are still contested" in Syria.

The battered city of Homs is "lost for now," but other cities are not, McCain said.

"Time is running out. Assad's forces are on the march," McCain said. The only realistic way to stop them, he said, was with foreign airpower, adding that this would require the United States to "suppress" Syrian air defenses in at least part of the country.

The Obama administration has to date stressed seeking a political solution to the Syrian crisis. Last month, however, the White House said it did not rule out "additional measures" if a political solution turned out to be impossible.

"These safe havens (in Syria) could serve as platforms for the delivery of humanitarian and military assistance - including weapons and ammunition, body armor and other personal protective equipment, tactical intelligence, secure communications equipment, food and water and medical supplies," said McCain, the ranking Republican on the Senate Armed Services Committee.

He said the safe havens could also help the Free Syrian Army and other armed groups in Syria train and organize themselves into more cohesive and effective military forces, "likely with the assistance of foreign partners."

While there would be no UN Security Council resolution on Syria because of Russian and Chinese opposition, McCain said, the US should seek active involvement of key Arab partners such as Saudi, Jordan, Qatar and the UAE as well as Nato allies such as Turkey.-Reuters