The United States must show stronger international leadership to halt the violence in Syria, which could become a significant issue in this year's presidential campaign, two U.S. senators said Thursday.

Republican Sen. John McCain said last weekend's massacre of dozens of Syrian civilians proved that "it's time to act; it's time to give the Syrian opposition the weapons in order to defend themselves."

"It is also embarrassing that the United States of America refuses to show leadership and come to the aid of the Syrian people," McCain told reporters while visiting Kuala Lumpur with independent Sen. Joe Lieberman at the invitation of a Malaysian business association.

Lieberman said he believed it might become necessary to use "the kind of American and allied airpower" against Syrian President Bashar Assad's forces as what was used to oust Libya's Moammar Gadhafi last year.

President Barack Obama's administration has so far said it remains opposed to military action in Syria. It has relied heavily on economic sanctions as a means for pressing Assad to leave power.

In the wake of last weekend's massacre of 108 civilians, including children, in the town of Houla, the U.S. and several other countries expelled Syrian diplomats to protest the killings.

McCain said he expects "significant debate" before the presidential election about "America's role in the world, about our leadership, about our commitment to the fundamental rights."

"I think a lot of Americans are asking the question, why are we standing by while innocent children are being murdered ... and we use nothing but rhetoric in response," he said.

Lieberman said he hopes it becomes a campaign issue because "America should judge itself by the extent to which we follow a foreign policy that has a moral and humanitarian component to it."

McCain and Lieberman also said they believe that a strong U.S. political and military commitment in Asia is needed to ensure regional stability and to help peacefully resolve the territorial dispute over the resource-rich South China Sea.

Both stressed they do not envision any serious confrontation between the U.S. and China, but added that they oppose Beijing's expansive claims over most of the South China Sea, which are disputed by Brunei, Malaysia, Taiwan, Vietnam and the Philippines.

"We support the position taken by the (Southeast Asian) countries that there be multilateral negotiations between China and the affected countries — not, as the Chinese state, on a one-by-one basis. That would obviously give an unfair advantage to China," McCain said.

Lieberman added that the U.S. was "not simply going to roll over and accept anything that China asserts."

"It's been a bedrock principle of American foreign policy to protect the freedom of the seas," he said.