But with Washington and Tokyo finalizing the deal late Thursday night, that cooperation could be a case of too little, too late.
On Tuesday, a bipartisan group of senators vowed to block any funding for the new plan, which moves half of the 18,000-strong Marine Corps force from Okinawa to Guam and elsewhere in the Pacific.
Some of the Marines shipped off Okinawa could end up at the Corps' newest outpost in Darwin, Australia, the official said during the briefing at the Pentagon.
Regardless of the new deal, "no new basing proposal can be considered final" until the Pentagon provides Capitol Hill with a concise and detailed cost analysis of the move, Sens. Carl LevinCarl LevinSilencing of Warren another example of hyperpartisan Senate GOP going nuclear over Gorsuch might destroy filibuster forever Obama to preserve torture report in presidential papers MORE (D-Mich.), John McCainJohn McCainHow does placing sanctions on Russia help America? THE MEMO: Trump's wild first month Trump’s feud with the press in the spotlight MORE (R-Ariz.) and Jim Webb (D-Va.) said in a letter sent to the Pentagon on Tuesday.
Levin, McCain and others requested DOD have that cost analysis ready for Congress by March. Department officials have yet to submit that analysis to Capitol Hill.
Senators initially wanted the cost information ready before any deal with Japan over Okinawa was reached.
Initial Defense assessments have the move costing the department a total of $8.6 billion, the defense official said Thursday. Japan has agreed to contribute $3.1 billion toward the estimated cost.
That cost will cover the build up of new facilities in Guam to house the incoming Marines and to build training ranges in Guam and the nearby Marianas Islands, according to the official.
The official noted, however, that the $8.6 billion price tag is a "preliminary estimate that will be refined" based on a "very rough order of magnitude estimates" for the move.
That said, "the senators have legitimate questions about cost, and we will consult with them closely as ... we go forward with this plan," the official said.
The State Department official declined to go into detail regarding those consultations during the same Pentagon briefing Thursday.
"We have a huge support from members of Congress who believe in what we're trying to accomplish in Asia, and we want to continue working with them moving forward as well," the official said.