WASHINGTON -- The Pentagon has no current plans to increase the size of the U.S. nuclear arsenal. In fact, it can barely sustain the existing force, which is decades old and is in some respects almost decrepit.

The arsenal is far from being in the "perfect shape" that President Donald Trump said Wednesday he wants to see under his watch. That is why the government is planning to spend hundreds of billions of dollars on a top-to-bottom "modernization," or replacement of the three major categories of nuclear weapons -- as well as their command and control systems -- in coming decades.

Those new weapons would replace, not add to, currently deployed forces such as the 400 Minuteman 3 intercontinental ballistic missiles that stand ready for short-notice launch in underground silos in North Dakota, Montana, Colorado, Wyoming and Nebraska.

Trump was asked during an Oval Office photo shoot whether he sought a big increase in the size of the nuclear force, as NBC News reported.

"No, I never discussed increasing it," he said. "I want it in perfect shape." He suggested he thinks the U.S. already has enough weapons. "We don't need an increase, but I want modernization and I want total rehabilitation," he said, apparently referring to replacing weapons and support systems that have grown old.

"I want to have absolutely perfectly maintained -- which we are in the process of doing -- nuclear force," he said. "But when they said I want 10 times what we have right now, it's totally unnecessary."

An in-depth review of the U.S. nuclear force and the strategies and polices that underpin it has been under way since April. The study, ordered by Trump and known as a nuclear "posture" review, is unlikely to be completed and made public before the end of the year, but it already is steering away from any major buildup in the size of the arsenal, officials familiar with the discussions say.

Instead, the focus is on maintaining the basic shape of a modernization plan Trump inherited from President Barack Obama, with possible adjustments, and on ways to reverse a long decline in the Energy Department's ability to build and sustain nuclear warheads, according to several officials who spoke on condition of anonymity to discuss internal deliberations.

The Pentagon review also is looking at the possibility of developing lower-yield nuclear weapons that proponents say would give the president additional options for responding to nuclear threats. Others say such weapons would make nuclear escalation more likely.

The U.S. has an estimated 4,000 nuclear weapons, of which about 1,800 are deployed on missiles and at bomber and fighter bases, according to Hans Kristensen, a nuclear weapons expert at the Federation of American Scientists. The others are held in reserve. The exact number of active and reserve weapons is an official secret.

The U.S. is constrained by a 2010 arms deal with Russia known as New START, which limits each country to a maximum of 1,550 deployed strategic nuclear warheads. As of Sept. 1, the U.S. reported that it had 1,393 and Russia had 1,561; both are required to be at or below the 1,550 mark by February 2018. That limitation will expire in 2021, however, unless an extension is negotiated.

Defence Secretary Jim Mattis took the unusual step Wednesday of issuing a public statement saying the NBC report that Trump had called for an increase in the nuclear arsenal was "absolutely false."

Public expectations about the direction of U.S. nuclear weapons policy are important because they can affect the credibility of U.S. commitments to arms control treaties and the durability of promises to U.S. allies like South Korea and Japan who count on protection under an American nuclear "umbrella." Some would argue that it also could influence the thinking of leaders like North Korea's Kim Jong Un, who sees his country as under siege from the United States and threatened by its long nuclear reach.

Trump's previous comments about nuclear weapons have caused confusion and concern in some quarters. Last December, for example, he suggested he favoured expanding the nuclear arsenal.

"The United States must greatly strengthen and expand its nuclear capability until such time as the world comes to its senses regarding nukes," he said then.

Trump has threatened to destroy North Korea should it attack the U.S. with a nuclear weapon.

The U.S. has no shortage of nuclear firepower, even if it has suffered recently from too few resources and in some cases a decline in morale among those responsible for operating and securing the weapons.

"I know the capability that we have, believe me, and it is awesome. It is massive," Trump said.