WASHINGTON: The United States has warned countries against supporting non-state actors and terrorist groups trying to obtain or employ nuclear weapons.

"The US will hold accountable any state or non-state actor that supports terrorist efforts to obtain or employ a nuclear weapon," under secretary of state for political affairs, Tom Shannon, told reporters at a Pentagon news conference where the Trump administration rolled out its 2018 Nuclear Posture Review (NPR).

Neither Shannon nor the NPR report, running into 100 pages, identified any country which could be seen aiding terrorist groups in this regard.

US officials have in the past expressed concern over the possibility of nuclear weapons of Pakistan landing into the hands of non-state actors or terrorist groups and thus has been working with Islamabad in enhancing the security of its nuclear weapons.

Pakistan has repeatedly denied such fears and asserted that its nuclear arsenals are one of the safest and secure ones in the world.

Shannon said that nuclear terrorism remained a major threat in the 21st century and countries need to work to mitigate it.

"The potential threat of non-state actors getting their hands on a nuclear weapon remains at the front of all of our minds," Shannon told reporters at a news conference held at the Pentagon.

Deputy energy secretary Dan Brouillette said the US will keep a close watch on nuclear proliferation activities.

"The department of energy will continue to focus on nonproliferation, counter proliferation and counter-terrorism. Together with our allies and partners, we have enjoyed great, great success in these areas," he said.

In a special section on nuclear terrorism, the report said the US strategy to combat nuclear terrorism encompasses a wide range of activities that comprise a defence-in-depth against current and emerging dangers.

The report said the US would try and prevent terrorists from obtaining nuclear weapons or technology and respond to nuclear incidents by locating and disabling a nuclear device or managing the consequences of a nuclear detonation.

"For effective deterrence, the US will hold fully accountable any state, terrorist group, or other non-state actor that supports or enables terrorist efforts to obtain or employ nuclear devices," the report said.

The report also said that a terrorist nuclear attack against the US or its allies and partners would qualify as an "extreme circumstance" under which the US could consider the "ultimate form of retaliation".

According to the report, the US will continue its efforts to minimise the number of nuclear weapons states, including by maintaining credible US-extended nuclear deterrence and assurance; and deny terrorist organisations access to nuclear weapons and materials.

It will also seek arms control agreements that enhance security, and are verifiable and enforceable.

The US will enhance cooperation with its allies, partners, and international institutions to combat nuclear terrorism, and deterring state support for nuclear terrorism through advanced forensics and attribution capabilities.

Noting that the nuclear Non-Proliferation Treaty (NPT) is the cornerstone of the nuclear non-proliferation regime, the report said that nuclear non-proliferation today faces acute challenges.

Most significantly, North Korea is pursuing a nuclear path in direct contravention of the NPT and in direct opposition to numerous UN Security Council resolutions, the report said.

The report also lists Iran as a challenge in terms of nuclear proliferation.

"Although the the Joint Comprehensive Plan of Action (JCPOA) may constrain Tehran's nuclear weapons program, there is little doubt Iran could achieve a nuclear weapon capability rapidly if it decides to do so," it said.

According to the report, the US will work to increase transparency and predictability to avoid potential miscalculation among nuclear weapons states and other possessor states.

"..through strategic dialogues, risk-reduction communications channels, and the sharing of best practices related to nuclear weapons safety and security," the report said.

In its report, the Trump administration said, although the US will not seek ratification of the Comprehensive Nuclear Test Ban Treaty, it will continue to support the Comprehensive Nuclear Test Ban Treaty Organization Preparatory Committee (CTBTO) as well as the International Monitoring System and the International Data Center.

"The US will not resume nuclear explosive testing unless necessary to ensure the safety and effectiveness of the US nuclear arsenal, and calls on all states possessing nuclear weapons to declare or maintain a moratorium on nuclear testing," it said.