Washington - The United States needs to strike Syria in part to send a message to its ally Iran over its nuclear programme, President Barack Obama's national security adviser said on Monday.
Susan Rice, joining a major public effort by Obama to persuade a sceptical Congress, said the United States was morally bound to respond to Syrian President Bashar Assad's alleged use of chemical weapons.
Rice said that US action on Syria was also critical for the broader influence of the United States, which has joined Israel and European nations in warning Iran against developing nuclear weapons.
"We will not allow Iran to acquire a nuclear weapon," Rice said at the New America Foundation, a think tank.
"As the president has said, all options remain on the table. For our efforts to succeed, however, the leaders in Tehran must know that the United States means what we say," she said.
"If we do not respond when Iran's close ally uses weapons of mass destruction, what message does that send to Iran? It risks suggesting that the international community cannot muster the will to act when necessary," Rice said.
Iran denies that it is seeking nuclear weapons, saying its programme is for peaceful purposes. Newly elected President Hassan Rowhani has called for better relations with the rest of the world but has strongly opposed a strike on Syria.
Iran, led by Shi'ite clerics, counts Assad - a secular leader from the minority Alawite sect - as its closest ally in the region, while the rebels are supported by Gulf Arab monarchies Qatar and Saudi Arabia.
Obama has resisted involvement in Syria's civil war, which has claimed more than 110 000 lives, but warned Assad that the use of chemical weapons was a "red line" for the United States.
Rice noted that Iran itself suffered chemical weapons attacks during the 1980-88 Iran-Iraq War. Iraqi leader Saddam Hussein, who launched the attacks, enjoyed Western assistance at the time.