Senators are getting a new round of briefings from the administration, including a top State Department official meeting with Senate Republicans during a closed-door lunch on Tuesday.

The letter to Trump comes as the Senate is expected to start debating the National Defense Authorization Act (NDAA) later this week.

"No funds may be used to conduct hostilities against the government of Iran, against the armed forces of Iran or in the territory of Iran except pursuant to an act or a joint resolution of Congress specifically authorizing such hostilities," the amendment reads.