Syrian opposition activists are reporting continued attacks by government forces in the northern city of Aleppo Monday, as prominent U.S. lawmakers call for deeper involvement from Washington.

The Syrian Observatory for Human Rights says the army is shelling several neighborhoods in Aleppo, and that at least two civilians have been killed.

The country's biggest city has become a key battleground in the uprising against President Bashar al-Assad's rule, as government forces are reported to have massed, ahead of what is expected to be a strong offensive against the opposition. Government and opposition claims in Syria are difficult to verify because journalists do not have a freedom of movement.

Meanwhile, U.S. senators John McCain, Joseph Lieberman and Lindsey Graham say the United States should provide direct assistance to the opposition, including weapons, intelligence and training.

In a joint opinion piece published late Sunday by the Washington Post, they say that the U.S. is jeopardizing its national security as well as its moral standing in the world by remaining on the sidelines.

The senators compared not aiding the rebels to the U.S. relationship with Kurds and Shi'ites in Iraq under Saddam Hussein and the Tutsis in Rwanda, saying inaction would haunt the nation “for years to come.”

U.S. Secretary of State Hillary Clinton is expected to visit Turkey next Saturday to discuss the deteriorating situation in Syria.