Two top senators on defense issues say U.S. Navy leaders should consider delaying deployment of the new Littoral Combat Ship and tone down their effusive rhetoric about the vessel until it successfully completes more testing.

With six ships of a planned 40 delivered but "practically no LCS mission capabilities proven" for mine clearance, surface warfare and submarine-hunting, "we urge you to reevaluate the deployment strategy," John McCain, chairman of the Senate Armed Services Committee, and Jack Reed, the panel's top Democrat, wrote in a letter dated Feb. 5.

As part of the Obama administration's promised "pivot to the Pacific," the Navy has sent one Littoral Combat Ship to Singapore with plans to have a total of two there by December and four by 2018. The vessel already in place, the USS Fort Worth, has been sidelined since a major crew-caused maintenance failure in January. The Navy also plans to use littoral ships, intended for operations in shallow coastal waters, in Pacific exercises and for a deployment to Bahrain in 2017.

Defense Secretary Ashton Carter and his predecessor, Chuck Hagel, both questioned the ship's ability to survive in combat. They cut the numbers to be purchased from the contractors that build separate versions, Lockheed Martin Corp. and Austal Ltd.

In the letter to Navy Secretary Ray Mabus and Adm. John Richardson, chief of naval operations, McCain and Reed also said they were "concerned that Navy leaders are overstating the current state of the program and the challenging path to achieving the promised capability."

"We expect Navy leaders to acknowledge and close the chasm between aspirations and reality for the LCS," they wrote.

The senators contrasted comments made by Mabus in January about the ship and its "robust" capabilities -- "now that's a success story," he said in a speech -- with a more critical assessment this month from the Pentagon's chief weapons tester that disclosed new questions about the LCS's combat effectiveness and reliability.

"We seldom hear from Navy leaders about these challenges," wrote McCain of Arizona and Reed of Rhode Island.

There's no timeline for declaring the Littoral Combat Ship combat-ready for mine-clearing, its top mission, the senators wrote. In addition, they said,"we are still years away from an LCS anti-submarine warfare" initial capability.

Asked about the letter, Rear Adm. Dawn Cutler, the Navy's chief spokeswoman, said, "We will continue to refine how we maintain, operate and deploy LCS based on what we learn in operational tests, maintenance and deployments."

"The first two deployments of LCS have been successful, but we still have work to do in order to better execute the mission for which this platform was designed," she said in an email.

In December, Carter directed the Navy to cut to 40 ships from 52 a planned mix of original LCS vessels and upgraded models that would be better-armed. Twenty-six vessels are now under contract in a $23 billion program.

In its annual report on major weapons, the Pentagon testing office disclosed that a Littoral Combat Ship had difficulty in tests at sea defending against swarming vessels, like those used by Iran's Islamic Revolutionary Guards, and that both versions of the ship have extensive reliability problems.

Given these conclusions, the lawmakers cautioned Navy leaders to dial back the praise and give a more realistic assessment of the vessel.

In his Jan. 14 speech to the Surface Navy Association, a Navy industry support group, Mabus said that "a group of small surface ships like the LCS is still capable of putting the enemy fleet at the bottom of the ocean."

McCain and Reed said they doubted that. While the ship's surface warfare system has been declared as having an initial capability, they said it's two years late, has "significant unresolved deficiencies," and still lacks long-range missiles to take out enemies at extended ranges.

"Unless the enemy fleet consists of a small number of lightly armed boats at extremely close range we fail to see how the LCS reality is consistent with the secretary's remarks," they wrote.