The president's new national security adviser, John Bolton, urged President Donald Trump to skip his trip to South America this week because of the ongoing crisis in Syria. The White House announced Tuesday that he would do so.

This reflects a view in the White House that deeper Russian and Iranian involvement in Syria have complicated calculations about a response to any U.S. military attack. That's according to an official who spoke on condition of anonymity because they were not authorized to discuss internal deliberations.

Moscow has cautioned the U.S. not to launch a military attack.