President Barack Obama is meeting with members of Congress as part of his bid to get approval from lawmakers to launch a military strike against Syria.
Secretary of State John Kerry, Defense Secretary Chuck Hagel and the Chairman of the Joint Chiefs of Staff General Martin Dempsey are due to appear later before the Senate Foreign Relations Committee.
White House officials are continuing to hold closed-door sessions briefing lawmakers in a bid to rally support for Mr. Obama's proposal.
On Monday, Senators John McCain and Lindsey Graham said they were encouraged after meeting with the president, but wanted to see some changes to the president's proposal.
McCain expressed the need to weaken Syrian President Bashar al-Assad's capabilities while upgrading those of the rebels opposing him.
The White House says Mr. Obama and Japan's Prime Minister Shinzo Abe discussed their "grave concern" about Syria's alleged use of chemical weapons during a phone call late Monday.
In a statement, the White House says the two leaders agreed that a use of chemical weapons was a "serious violation of international norms" and could not be tolerated.
U.N. Secretary-General Ban Ki-moon will brief members of the Security Council, on Tuesday, about the latest developments in Syria.
Mr. Ban has met with the head of a U.N. inspection team that was in Syria last week collecting samples from the chemical attack last month on civilians near Damascus. That attack killed at least 1,400 people. The samples were sent to labs on Monday for analysis.
Mr. Assad denies his military was responsible for the use of chemical weapons, saying the United States and France have no proof to back up their allegations.