Four U.S. senators said on Tuesday that the UN Security Council should refer credible charges of crimes against humanity by Syrian President Bashar Assad to the International Criminal Court.

AFP reported that the Democratic senators have sent a letter to U.S. Ambassador to the UN, Susan Rice, in which they wrote that it was time for the ICC to look into “deeply troubling and grave charges” against Assad amid his government’s bloody crackdown on protesters.

The senators signed to the letter are Dick Durbin, Benjamin Cardin, Robert Menendez and Barbara Boxer, AFP said.

“It is paramount that the Security Council refers credible allegations of crimes against humanity by President Bashar al-Assad’s regime to the International Criminal Court,” they wrote, adding: “The people of Syria deserve to know that the people of the United States understand their plight, stand behind them, and will work to bring justice to their country.”

At least 3,000 Syrian citizens have been killed in Assad’s ongoing crackdown against anti-regime protesters.

Earlier this week, the United States told its ambassador to Syria, Robert Ford, to leave the country because of fears for his safety, following more anti-American incitement and several attacks on his residence.

Ford placed himself in danger in July when he traveled to Hama and Deraa, two centers of anti-Assad protests. He later paid a condolence call on the home of a protest leader who was arrested and apparently tortured to death by Assad’s soldiers and goon squads.

In their letter, the senators addressed Ford’s leaving the country, saying this “should also be deeply troubling” to the UN Security Council.

Meanwhile, Senator John McCain said on Monday that NATO might have to move from Libya to Syria now that Qaddafi is gone.

“Now that military operations in Libya are ending, there will be renewed focus on what practical military operations might be considered to protect civilian lives in Syria,” McCain said at the World Economic Forum in Jordan.

“The Assad regime should not consider that it can get away with mass murder,” he added. “Qaddafi made that mistake and it cost him everything. Iran’s rulers would be wise to heed similar counsel.”