The Comfort is a seagoing medical treatment facility "capable of providing resuscitation and stabilization care; initial wound and basic surgery; and postoperative treatment," according to the US Navy.
Puerto Rican officials asked for additional medical assistance from the ship after initially indicating that they wanted to focus on getting power restored to damaged hospitals on shore, US officials told CNN.
While the Comfort is on the way, according to FEMA administrator Brock Long, it may not deploy for another week -- within five to nine days as additional forces are activated to help man the ship along with its current crew of about 18 civilian mariners.
The crew is expected to grow to as many 60 and the medical crew could increase from the approximately 65 now with the ship to as many as 1,200, according to officials.
Hillary Clinton urged the Defense Department to send a Navy medical ship, while Democratic House Minority Leader Nancy Pelosi called for Republicans to join with Democrats to pass a robust relief package.
The Navy hospital ship is just one part of the military's humanitarian assistance effort -- deploying assets to both Puerto Rico and the US Virgin Islands after the storm decimated critical infrastructure in the region.
The USS Kearsarge Amphibious Ready Group has already been conducting rescue operations in the region, including eight medical evacuations and 148 airlifts, and delivered 44,177 pounds of relief supplies and cargo to Puerto Rico and the US Virgin Islands since Maria struck, according to the Pentagon.
The US Air Force is also sending additional aircraft to Puerto Rico and the Virgin Islands to ramp up the volume of daily relief missions.
Speaking on Capitol Hill Tuesday, Chairman of the Joint Chiefs Gen. Joseph Dunford said the priority is making sure airfields can operate.
"The responsibility is primarily DHS, but at this point we're not trapped in bureaucratic niceties, what we're trying to do is make sure that we get the people of Puerto Rico the support they need when they need it," he said.
"All the other support they need can't come in until we get the ports and airfields open, so that's why Northern Command has placed that at the top of the list in terms of the support we're providing," he added.
Dunford said he expects more military aircraft will be used particularly for "generators, water, food, those kind of immediate needs."
Priorities for Puerto Rico
In addition to deploying the USNS Comfort for additional medical assistance, the Defense Department is supporting FEMA's immediate priorities for Puerto Rico including: debris and route clearance of critical transportation routes; re-opening air and sea ports; re-establishing utilities, and implementation of a commodities distribution plan.
"Northern Command continues to conduct 24-hour operations, aggressively conducting search and rescue operations, bringing additional essential commodities to the islands, and restoring power at hospitals, ports, airports and other critical facilities," according to the service.
The US military is sending a variety of additional aircraft vehicles to assist with ongoing missions aimed to deliver critical supplies and capabilities to Puerto Rico:
One C-17 Globemaster with the remaining Contingency Response Element to establish airport operations. This unit includes more than 50 personnel.
One C-17 and one C-5 with the Mobile Emergency Response Support unit to provide mobile telecommunications, life support, logistics, operational support and power generation.
One C-17 with the 3rd Expeditionary Sustainment Command from Fort Bragg, North Carolina. This unit includes approximately 20 personnel with tactical trucks and utility trailers to provide site assessments.
One C-17 with the 63d Signal Battalion from Fort Stewart, Georgia. This unit includes approximately 15 personnel, tactical trucks and utility trailers to provide satellite ground communications capabilities.
Two C-5s with the 101st Combat Aviation Brigade from Fort Campbell, Kentucky, to provide tactical airlift with eight helicopters.
Two C-130s with the FAA's Incident Staging Base Cache from Dobbins Air Reserve Base, Georgia. This unit includes approximately 50 personnel and four heavy duty trucks and equipment.
The Air Force is also further increasing airport throughput capacity in San Juan by assisting the Federal Aviation Administration with air traffic control repairs.
Priorities for US Virgin Islands
The US military is also deploying additional aerial and medical assets to the Virgin Islands to assist in ongoing relief efforts:
Four C-17s with water and meals to St Croix.
Two C-17s and one C-130 to St Thomas with National Guard units that include approximately 120 personnel.