WASHINGTON — The UN Security Council should refer credible charges of crimes against humanity by Syrian President Bashar al-Assad to the International Criminal Court (ICC), US senators urged Tuesday.

In a letter to Washington's UN envoy, Susan Rice, the lawmakers said it was time for the ICC to look into "deeply troubling and grave charges" against Assad amid his government's bloody crackdown on protestors.

"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.

Democratic Senators Dick Durbin, his party's number-two in the chamber, as well as Benjamin Cardin, Robert Menendez and Barbara Boxer signed the letter to Rice.

According to UN estimates, more than 3,000 people have been killed in the crackdown against dissent since protests erupted in Syria in mid-March amid an "Arab Spring" of demonstrations across the region.

"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," the senators wrote.

The lawmakers noted that US Ambassador to Syria Robert Ford, an open critic of the crackdown, left Syria "due to credible threats to his safety" and said this "should also be deeply troubling" to the UN Security Council.