KABUL, Afghanistan — With the Syrian government continuing its deadly crackdown on its citizens, two senior American senators who were on their way to the Middle East spoke out strongly on Sunday in favor of arming the Syrian opposition forces.

The senators, John McCain of Arizona and Lindsey Graham of South Carolina, both Republicans, laid out a series of diplomatic, humanitarian and military aid proposals that would put the United States squarely behind the effort to topple President Bashar al-Assad of Syria. The senators, both of whom are on the Senate Armed Services Committee, said that rebel fighters deserved to be armed and that helping them take on the Syrian government would aid Washington’s effort to weaken Iran.

Syria relies on Iran for financial and military support, and the governments in Damascus and Tehran have sectarian ties as well: Iran has strongly backed the Syrian Shiite minority and the offshoot Alawite sect that makes up Syria’s ruling class.

“I believe there are ways to get weapons to the opposition without direct United States involvement,” Mr. McCain said. “The Iranians and the Russians are providing Bashar Assad with weapons. People that are being massacred deserve to have the ability to defend themselves.”