The Navy is set to get back into U.S. Southern Command’s growing and “vital” anti-drug mission, dedicating at least four ships to the effort in 2018, according to a Navy secretary letter obtained by Navy Times.

The Navy stopped providing surface combatants for the anti-drug effort in 2015, when the service retired its last Oliver Hazard Perry-class frigate, although patrol coastal ships have operated in the command’s waters since then.

Secretary Richard Spencer’s Dec. 5 memo to Chief of Naval Operations Adm. John Richardson states that SOUTHCOM’s joint task force “is a very important element in the control of illegal drug transportation.”

“Since then maritime events have doubled and more growth is expected by the Task Force in the coming years,” Spencer said in the letter. “Clearly the presence of Navy ships had a deterrent effect on the drug transportation process. We must restore this impact now in this vital national priority program.”

The four ships the Navy will provide are “well below the SOUTHCOM requested amount,” Spencer said.

While Navy leadership this year bandied about the idea of reactivating the mothballed Oliver Hazard Perry-class frigate to help build the fleet, Spencer said the ships won’t be resurrected for this mission.

Instead SOUTHCOM will receive the littoral combat ship and Spearhead-class expeditionary fast transport vessels, the letter states.

The LCS program has come under fire for cost overruns, too little armor and other shortcomings, but Spencer’s letter states the ship is “ideally configured for this low intensity operation.”

The fast transport ships have “significant potential with some minor changes” for SOUTHCOM’s interdiction needs, the letter states.

“This mission must be in the top priority category for these ships, reversing the prior decision to eliminate support,” the letter states.

In a time of tight budgets and heavy demand for Navy ships, Spencer noted that restarting the Navy’s part in the SOUTHCOM task force should be budgeted for.

Earlier this year, Spencer mentioned the Perry frigates as a low-cost alternative to help with drug interdiction operations, and Richardson pointed to their resurrection as a cost-effective way to help get the service to its 355-ship goal.

But an internal document obtained by Defense News this fall showed that bringing back the ships would cost billions, cut into modernization accounts for other ships and add little to Navy capabilities.

Spencer’s office did not immediately clarify whether the Perry frigates could still be resurrected for other missions.